diff --git a/browser/base/content/test/general/browser_bug676619.js b/browser/base/content/test/general/browser_bug676619.js
index 8950709b8c78..a57cbfb9ab01 100644
--- a/browser/base/content/test/general/browser_bug676619.js
+++ b/browser/base/content/test/general/browser_bug676619.js
@@ -75,9 +75,9 @@ async function runTest(url) {
 
   await testLink("link1", "test.txt");
   await testLink("link2", "video.ogg");
-  await testLink("link3", "just some video.ogg");
+  await testLink("link3", "just some video");
   await testLink("link4", "with-target.txt");
-  await testLink("link5", "javascript.html");
+  await testLink("link5", "javascript.txt");
   await testLink("link6", "test.blob");
   await testLink("link7", "test.file");
   await testLink("link8", "download_page_3.txt");
@@ -86,11 +86,6 @@ async function runTest(url) {
   await testLink("link11", "download_page_4.txt");
   await testLocation("link12", "http://example.com/");
 
-  // Check that we enforce the correct extension if the website's
-  // is bogus or missing:
-  await testLink("link13", "no file extension.ogg");
-  await testLink("link14", "javascript.html");
-
   BrowserTestUtils.removeTab(tab);
 }
 
diff --git a/browser/base/content/test/general/download_page.html b/browser/base/content/test/general/download_page.html
index e6cba990cbd1..802d13e633ac 100644
--- a/browser/base/content/test/general/download_page.html
+++ b/browser/base/content/test/general/download_page.html
@@ -16,11 +16,11 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=676619
         <li><a href="video.ogg"
                 download id="link2">Download "video.ogg"</a></li>
         <li><a href="video.ogg"
-                download="just some video.ogg" id="link3">Download "just some video.ogg"</a></li>
+                download="just some video" id="link3">Download "just some video"</a></li>
         <li><a href="download_page_2.txt"
                 download="with-target.txt" id="link4">Download "with-target.txt"</a></li>
         <li><a href="javascript:(1+2)+''"
-            download="javascript.html" id="link5">Download "javascript.html"</a></li>
+            download="javascript.txt" id="link5">Download "javascript.txt"</a></li>
         <li><a href="#" download="test.blob" id=link6>Download "test.blob"</a></li>
         <li><a href="#" download="test.file" id=link7>Download "test.file"</a></li>
         <li><a href="download_with_content_disposition_header.sjs?inline=download_page_3.txt"
@@ -33,19 +33,15 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=676619
             download="download_page_4.txt" id="link11">Download "download_page_4.txt"</a></li>
         <li><a href="http://example.com/"
                 download="example.com" id="link12" target="_blank">Download "example.com"</a></li>
-        <li><a href="video.ogg"
-                download="no file extension" id="link13">Download "force extension"</a></li>
-        <li><a href="javascript:(1+2)+'force extension'"
-            download="javascript.txt" id="link14">Download "javascript.txt" (will correct extension due to mimetype)</a></li>
     </ul>
     <div id="unload-flag">Okay</div>
 
     <script>
-        let blobURL = window.URL.createObjectURL(new Blob(["just text"], {type: "application/x-blob"}));
+        let blobURL = window.URL.createObjectURL(new Blob(["just text"]));
         document.getElementById("link6").href = blobURL;
 
         let fileURL = window.URL.createObjectURL(new File(["just text"],
-              "wrong-file-name", {type: "application/x-some-file"}));
+                                                          "wrong-file-name"));
         document.getElementById("link7").href = fileURL;
 
         window.addEventListener("beforeunload", function(evt) {
diff --git a/browser/components/downloads/test/browser/browser.ini b/browser/components/downloads/test/browser/browser.ini
index ac7ebdef9d29..15fdd9a632f7 100644
--- a/browser/components/downloads/test/browser/browser.ini
+++ b/browser/components/downloads/test/browser/browser.ini
@@ -10,10 +10,6 @@ support-files =
   !/toolkit/content/tests/browser/common/mockTransfer.js
 [browser_first_download_panel.js]
 skip-if = os == "linux" # Bug 949434
-[browser_image_mimetype_issues.js]
-support-files =
-  not-really-a-jpeg.jpeg
-  not-really-a-jpeg.jpeg^headers^
 [browser_overflow_anchor.js]
 skip-if = os == "linux" # Bug 952422
 [browser_confirm_unblock_download.js]
diff --git a/browser/components/downloads/test/browser/browser_image_mimetype_issues.js b/browser/components/downloads/test/browser/browser_image_mimetype_issues.js
deleted file mode 100644
index f5231329a174..000000000000
--- a/browser/components/downloads/test/browser/browser_image_mimetype_issues.js
+++ /dev/null
@@ -1,113 +0,0 @@
-/* Any copyright is dedicated to the Public Domain.
-   http://creativecommons.org/publicdomain/zero/1.0/ */
-
-"use strict";
-
-const TEST_ROOT = getRootDirectory(gTestPath).replace(
-  "chrome://mochitests/content",
-  "http://example.com"
-);
-
-var MockFilePicker = SpecialPowers.MockFilePicker;
-MockFilePicker.init(window);
-
-/*
- * Popular websites implement image optimization as serving files with
- * extension ".jpg" but content type "image/webp". If we save such images,
- * we should actually save them with a .webp extension as that is what
- * they are.
- */
-
-/**
- * Test the above with the "save image as" context menu.
- */
-add_task(async function test_save_image_webp_with_jpeg_extension() {
-  await BrowserTestUtils.withNewTab(
-    `data:text/html,<img src="${TEST_ROOT}/not-really-a-jpeg.jpeg?convert=webp">`,
-    async browser => {
-      let menu = document.getElementById("contentAreaContextMenu");
-      let popupShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
-      BrowserTestUtils.synthesizeMouse(
-        "img",
-        5,
-        5,
-        { type: "contextmenu", button: 2 },
-        browser
-      );
-      await popupShown;
-
-      await new Promise(resolve => {
-        MockFilePicker.showCallback = function(fp) {
-          ok(
-            fp.defaultString.endsWith("webp"),
-            `filepicker for image has "${fp.defaultString}", should end in webp`
-          );
-          setTimeout(resolve, 0);
-          return Ci.nsIFilePicker.returnCancel;
-        };
-        EventUtils.synthesizeMouseAtCenter(
-          menu.querySelector("#context-saveimage"),
-          {}
-        );
-      });
-    }
-  );
-});
-
-/**
- * Test with the "save link as" context menu.
- */
-add_task(async function test_save_link_webp_with_jpeg_extension() {
-  await BrowserTestUtils.withNewTab(
-    `data:text/html,<a href="${TEST_ROOT}/not-really-a-jpeg.jpeg?convert=webp">Nice image</a>`,
-    async browser => {
-      let menu = document.getElementById("contentAreaContextMenu");
-      let popupShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
-      BrowserTestUtils.synthesizeMouse(
-        "a[href]",
-        5,
-        5,
-        { type: "contextmenu", button: 2 },
-        browser
-      );
-      await popupShown;
-
-      await new Promise(resolve => {
-        MockFilePicker.showCallback = function(fp) {
-          ok(
-            fp.defaultString.endsWith("webp"),
-            `filepicker for link has "${fp.defaultString}", should end in webp`
-          );
-          setTimeout(resolve, 0);
-          return Ci.nsIFilePicker.returnCancel;
-        };
-        EventUtils.synthesizeMouseAtCenter(
-          menu.querySelector("#context-savelink"),
-          {}
-        );
-      });
-    }
-  );
-});
-
-/**
- * Test with the main "save page" command.
- */
-add_task(async function test_save_page_on_image_document() {
-  await BrowserTestUtils.withNewTab(
-    `${TEST_ROOT}/not-really-a-jpeg.jpeg?convert=webp`,
-    async browser => {
-      await new Promise(resolve => {
-        MockFilePicker.showCallback = function(fp) {
-          ok(
-            fp.defaultString.endsWith("webp"),
-            `filepicker for "save page" has "${fp.defaultString}", should end in webp`
-          );
-          setTimeout(resolve, 0);
-          return Ci.nsIFilePicker.returnCancel;
-        };
-        document.getElementById("Browser:SavePage").doCommand();
-      });
-    }
-  );
-});
diff --git a/browser/components/downloads/test/browser/not-really-a-jpeg.jpeg b/browser/components/downloads/test/browser/not-really-a-jpeg.jpeg
deleted file mode 100644
index 04b7f003b463..000000000000
Binary files a/browser/components/downloads/test/browser/not-really-a-jpeg.jpeg and /dev/null differ
diff --git a/browser/components/downloads/test/browser/not-really-a-jpeg.jpeg^headers^ b/browser/components/downloads/test/browser/not-really-a-jpeg.jpeg^headers^
deleted file mode 100644
index c1a7794310b5..000000000000
--- a/browser/components/downloads/test/browser/not-really-a-jpeg.jpeg^headers^
+++ /dev/null
@@ -1,2 +0,0 @@
-Content-Type: image/webp
-
diff --git a/dom/security/test/general/browser_test_data_text_csv.js b/dom/security/test/general/browser_test_data_text_csv.js
index 8aae2f67e751..23a3e5576325 100644
--- a/dom/security/test/general/browser_test_data_text_csv.js
+++ b/dom/security/test/general/browser_test_data_text_csv.js
@@ -7,47 +7,46 @@ const kTestPath = getRootDirectory(gTestPath).replace(
 const kTestURI = kTestPath + "file_data_text_csv.html";
 
 function addWindowListener(aURL, aCallback) {
-  return new Promise(resolve => {
-    Services.wm.addListener({
-      onOpenWindow(aXULWindow) {
-        info("window opened, waiting for focus");
-        Services.wm.removeListener(this);
-        var domwindow = aXULWindow.docShell.domWindow;
-        waitForFocus(function() {
-          is(
-            domwindow.document.location.href,
-            aURL,
-            "should have seen the right window open"
-          );
-          resolve(domwindow);
-        }, domwindow);
-      },
-      onCloseWindow(aXULWindow) {},
-    });
+  Services.wm.addListener({
+    onOpenWindow(aXULWindow) {
+      info("window opened, waiting for focus");
+      Services.wm.removeListener(this);
+      var domwindow = aXULWindow.docShell.domWindow;
+      waitForFocus(function() {
+        is(
+          domwindow.document.location.href,
+          aURL,
+          "should have seen the right window open"
+        );
+        aCallback(domwindow);
+      }, domwindow);
+    },
+    onCloseWindow(aXULWindow) {},
   });
 }
 
-add_task(async function() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["security.data_uri.block_toplevel_data_uri_navigations", true]],
+function test() {
+  waitForExplicitFinish();
+  Services.prefs.setBoolPref(
+    "security.data_uri.block_toplevel_data_uri_navigations",
+    true
+  );
+  registerCleanupFunction(function() {
+    Services.prefs.clearUserPref(
+      "security.data_uri.block_toplevel_data_uri_navigations"
+    );
   });
-  let windowPromise = addWindowListener(
-    "chrome://mozapps/content/downloads/unknownContentType.xhtml"
+  addWindowListener(
+    "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+    function(win) {
+      is(
+        win.document.getElementById("location").value,
+        "text/csv;foo,bar,foobar",
+        "file name of download should match"
+      );
+      win.close();
+      finish();
+    }
   );
   BrowserTestUtils.loadURI(gBrowser, kTestURI);
-  let win = await windowPromise;
-
-  let expectedValue = "text/csv;foo,bar,foobar";
-  let mimeInfo = getMIMEInfoForType("text/csv");
-  try {
-    expectedValue = "." + mimeInfo.primaryExtension;
-  } catch (ex) {
-    /* fails on Windows, bug 1671930 */
-  }
-  is(
-    win.document.getElementById("location").value,
-    expectedValue,
-    "file name of download should match"
-  );
-  win.close();
-});
+}
diff --git a/toolkit/content/contentAreaUtils.js b/toolkit/content/contentAreaUtils.js
index 9c097440748d..6543efbbb019 100644
--- a/toolkit/content/contentAreaUtils.js
+++ b/toolkit/content/contentAreaUtils.js
@@ -1149,16 +1149,6 @@ function getDefaultExtension(aFilename, aURI, aContentType) {
     return "";
   } // temporary fix for bug 120327
 
-  // For images, rely solely on the mime type if known.
-  // All the extension is going to do is lie to us.
-  if (aContentType?.startsWith("image/")) {
-    let mimeInfo = getMIMEInfoForType(aContentType, "");
-    let exts = Array.from(mimeInfo.getFileExtensions());
-    if (exts.length) {
-      return exts[0];
-    }
-  }
-
   // First try the extension from the filename
   var url = Cc["@mozilla.org/network/standard-url-mutator;1"]
     .createInstance(Ci.nsIURIMutator)
diff --git a/uriloader/exthandler/nsExternalHelperAppService.cpp b/uriloader/exthandler/nsExternalHelperAppService.cpp
index 40b7a00bf957..8e38cbf6e3fa 100644
--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -203,7 +203,6 @@ static bool GetFilenameAndExtensionFromChannel(nsIChannel* aChannel,
   bool handleExternally = false;
   uint32_t disp;
   nsresult rv = aChannel->GetContentDisposition(&disp);
-  bool gotFileNameFromURI = false;
   if (NS_SUCCEEDED(rv)) {
     aChannel->GetContentDispositionFilename(aFileName);
     if (disp == nsIChannel::DISPOSITION_ATTACHMENT) handleExternally = true;
@@ -230,7 +229,6 @@ static bool GetFilenameAndExtensionFromChannel(nsIChannel* aChannel,
     nsAutoCString leafName;
     url->GetFileName(leafName);
     if (!leafName.IsEmpty()) {
-      gotFileNameFromURI = true;
       rv = UnescapeFragment(leafName, url, aFileName);
       if (NS_FAILED(rv)) {
         CopyUTF8toUTF16(leafName, aFileName);  // use escaped name
@@ -238,14 +236,14 @@ static bool GetFilenameAndExtensionFromChannel(nsIChannel* aChannel,
     }
   }
 
-  // If we have a filename and no extension, remove trailing dots from the
-  // filename and extract the extension if that is possible.
-  if (aExtension.IsEmpty() && !aFileName.IsEmpty()) {
-    // Windows ignores terminating dots. So we have to as well, so
-    // that our security checks do "the right thing"
-    aFileName.Trim(".", false);
+  // Extract Extension, if we have a filename; otherwise,
+  // truncate the string
+  if (aExtension.IsEmpty()) {
+    if (!aFileName.IsEmpty()) {
+      // Windows ignores terminating dots. So we have to as well, so
+      // that our security checks do "the right thing"
+      aFileName.Trim(".", false);
 
-    if (!gotFileNameFromURI || aAllowURLExtension) {
       // XXX RFindCharInReadable!!
       nsAutoString fileNameStr(aFileName);
       int32_t idx = fileNameStr.RFindChar(char16_t('.'));
@@ -1293,32 +1291,12 @@ nsExternalAppHandler::nsExternalAppHandler(
   mSuggestedFileName.CompressWhitespace();
   mTempFileExtension.CompressWhitespace();
 
-  // After removing trailing whitespaces from the name, if we have a
-  // temp file extension, replace the file extension with it if:
-  // - there is no extant file extension (or it only consists of ".")
-  // - the extant file extension contains invalid characters, or
-  // - the extant file extension is not known by the mimetype.
-  bool knownExtension = false;
-  // Note that originalFileExt is either empty or consists of an
-  // extension *including the dot* which we need to remove:
-  bool haveBogusExtension =
-      mMimeInfo && !originalFileExt.IsEmpty() &&
-      NS_SUCCEEDED(mMimeInfo->ExtensionExists(
-          Substring(NS_ConvertUTF16toUTF8(originalFileExt), 1),
-          &knownExtension)) &&
-      !knownExtension;
-  if (!mTempFileExtension.IsEmpty() &&
-      (originalFileExt.Length() == 0 || originalFileExt.EqualsLiteral(".") ||
-       originalFileExt.FindCharInSet(
-           KNOWN_PATH_SEPARATORS FILE_ILLEGAL_CHARACTERS) != kNotFound ||
-       haveBogusExtension)) {
-    int32_t pos = mSuggestedFileName.RFindChar('.');
-    if (pos != kNotFound) {
-      mSuggestedFileName =
-          Substring(mSuggestedFileName, 0, pos) + mTempFileExtension;
-    } else {
-      mSuggestedFileName.Append(mTempFileExtension);
-    }
+  // Append after removing trailing whitespaces from the name.
+  if (originalFileExt.FindCharInSet(
+          KNOWN_PATH_SEPARATORS FILE_ILLEGAL_CHARACTERS) != kNotFound) {
+    // The file extension contains invalid characters and using it would
+    // generate an unusable file, thus use mTempFileExtension instead.
+    mSuggestedFileName.Append(mTempFileExtension);
     originalFileExt = mTempFileExtension;
   }
 
diff --git a/uriloader/exthandler/tests/mochitest/test_invalidCharFileExtension.xhtml b/uriloader/exthandler/tests/mochitest/test_invalidCharFileExtension.xhtml
index 4ee1a6a1c114..accbb5dec59a 100644
--- a/uriloader/exthandler/tests/mochitest/test_invalidCharFileExtension.xhtml
+++ b/uriloader/exthandler/tests/mochitest/test_invalidCharFileExtension.xhtml
@@ -9,9 +9,9 @@
 <iframe id="test"></iframe>
 <script type="text/javascript">
 var tests = [
-  ["test.png:large", "test.png"],
-  ["test.png/large", "test.png"],
-  [":test.png::large:", "test.png"],
+  ["test.png:large", "test.png large.png"],
+  ["test.png/large", "test.png_large.png"],
+  [":test.png::large:", "test.png large.png"],
 ];
 
 add_task(async function() {