diff --git a/image/imgLoader.cpp b/image/imgLoader.cpp
index 764d397093d8..aac684d12753 100644
--- a/image/imgLoader.cpp
+++ b/image/imgLoader.cpp
@@ -2729,6 +2729,8 @@ imgLoader::GetMIMETypeFromContent(nsIRequest* aRequest,
 nsresult imgLoader::GetMimeTypeFromContent(const char* aContents,
                                            uint32_t aLength,
                                            nsACString& aContentType) {
+  nsAutoCString detected;
+
   /* Is it a GIF? */
   if (aLength >= 6 &&
       (!strncmp(aContents, "GIF87a", 6) || !strncmp(aContents, "GIF89a", 6))) {
@@ -2781,8 +2783,9 @@ nsresult imgLoader::GetMimeTypeFromContent(const char* aContents,
     aContentType.AssignLiteral(IMAGE_WEBP);
 
   } else if (MatchesMP4(reinterpret_cast<const uint8_t*>(aContents), aLength,
-                        aContentType)) {
-    MOZ_ASSERT(aContentType.Equals(IMAGE_AVIF));
+                        detected) &&
+             detected.Equals(IMAGE_AVIF)) {
+    aContentType.AssignLiteral(IMAGE_AVIF);
   } else {
     /* none of the above?  I give up */
     return NS_ERROR_NOT_AVAILABLE;
diff --git a/image/test/gtest/TestLoader.cpp b/image/test/gtest/TestLoader.cpp
index 286be2e7a7f6..289cebae9c01 100644
--- a/image/test/gtest/TestLoader.cpp
+++ b/image/test/gtest/TestLoader.cpp
@@ -87,6 +87,16 @@ TEST_F(ImageLoader, DetectAVIFCompatibleBrand) {
   CheckMimeType(buffer, sizeof(buffer), IMAGE_AVIF);
 }
 
+TEST_F(ImageLoader, DetectNonImageMP4) {
+  const char buffer[] =
+      "\x00\x00\x00\x1c"  // box length
+      "ftyp"              // box type
+      "isom"              // major brand
+      "\x00\x00\x02\x00"  // minor version
+      "isomiso2mp41";     // compatible brands
+  CheckMimeType(buffer, sizeof(buffer), nullptr);
+}
+
 TEST_F(ImageLoader, DetectNone) {
   const char buffer[] = "abcdefghijklmnop";
   CheckMimeType(buffer, sizeof(buffer), nullptr);