diff --git a/intl/uconv/public/nsIPlatformCharset.h b/intl/uconv/public/nsIPlatformCharset.h
index 9a070cb29ff7..08091eef8142 100644
--- a/intl/uconv/public/nsIPlatformCharset.h
+++ b/intl/uconv/public/nsIPlatformCharset.h
@@ -62,7 +62,8 @@ typedef enum {
      kPlatformCharsetSel_4xBookmarkFile = 3,
      kPlatformCharsetSel_KeyboardInput = 4,
      kPlatformCharsetSel_WindowManager = 5,
-     kPlatformCharsetSel_4xPrefsJS = 6
+     kPlatformCharsetSel_4xPrefsJS = 6,
+     kPlatformCharsetSel_PlainTextInFile = 7
 } nsPlatformCharsetSel;
 
 class nsIPlatformCharset : public nsISupports
diff --git a/mailnews/base/util/nsMsgI18N.cpp b/mailnews/base/util/nsMsgI18N.cpp
index ec8c6d324bdd..2d1eac3653f2 100644
--- a/mailnews/base/util/nsMsgI18N.cpp
+++ b/mailnews/base/util/nsMsgI18N.cpp
@@ -216,6 +216,21 @@ const char * nsMsgI18NFileSystemCharset()
 	return fileSystemCharset.get();
 }
 
+// Charset used by the text file.
+void nsMsgI18NTextFileCharset(nsACString& aCharset)
+{
+  nsresult rv;
+  nsCOMPtr <nsIPlatformCharset> platformCharset =
+    do_GetService(NS_PLATFORMCHARSET_CONTRACTID, &rv);
+  if (NS_SUCCEEDED(rv)) {
+    rv = platformCharset->GetCharset(kPlatformCharsetSel_PlainTextInFile,
+                                     aCharset);
+  }
+
+  if (NS_FAILED(rv))
+    aCharset.Assign("ISO-8859-1");
+}
+
 // MIME encoder, output string should be freed by PR_FREE
 // XXX : fix callers later to avoid allocation and copy
 char * nsMsgI18NEncodeMimePartIIStr(const char *header, PRBool structured, const char *charset, PRInt32 fieldnamelen, PRBool usemime) 
diff --git a/mailnews/base/util/nsMsgI18N.h b/mailnews/base/util/nsMsgI18N.h
index ca7b1bc167ed..ac4c21f87da2 100644
--- a/mailnews/base/util/nsMsgI18N.h
+++ b/mailnews/base/util/nsMsgI18N.h
@@ -105,6 +105,13 @@ NS_MSG_BASE const char *nsMsgI18NGetAcceptLanguage(void);
  */
 NS_MSG_BASE const char * nsMsgI18NFileSystemCharset(void);
 
+/**
+ * Return charset name of text file (OS dependent).
+ *
+ * @param aCharset    [OUT] Text file charset name.
+ */
+NS_MSG_BASE void nsMsgI18NTextFileCharset(nsACString& aCharset);
+
 /**
  * Convert from unicode to target charset.
  *
diff --git a/mailnews/compose/src/nsMsgCompose.cpp b/mailnews/compose/src/nsMsgCompose.cpp
index 2055fc5d0358..e9167ef8bfd5 100644
--- a/mailnews/compose/src/nsMsgCompose.cpp
+++ b/mailnews/compose/src/nsMsgCompose.cpp
@@ -3498,7 +3498,8 @@ nsMsgCompose::ConvertTextToHTML(nsFileSpec& aSigFile, nsString &aSigData)
 }
 
 nsresult
-nsMsgCompose::LoadDataFromFile(nsFileSpec& fSpec, nsString &sigData)
+nsMsgCompose::LoadDataFromFile(nsFileSpec& fSpec, nsString &sigData,
+                               PRBool aAllowUTF8)
 {
   PRInt32       readSize;
   PRInt32       nGot;
@@ -3534,9 +3535,17 @@ nsMsgCompose::LoadDataFromFile(nsFileSpec& fSpec, nsString &sigData)
   nsCAutoString sigEncoding(nsMsgI18NParseMetaCharset(&fSpec));
   PRBool removeSigCharset = !sigEncoding.IsEmpty() && m_composeHTML;
 
-  //default to platform encoding for signature files w/o meta charset
-  if (sigEncoding.IsEmpty())
-    sigEncoding.Assign(nsMsgI18NFileSystemCharset());
+  if (sigEncoding.IsEmpty()) {
+    if (aAllowUTF8 && IsUTF8(nsDependentCString(readBuf))) {
+      sigEncoding.Assign("UTF-8");
+    }
+    else {
+      //default to platform encoding for plain text files w/o meta charset
+      nsCAutoString textFileCharset;
+      nsMsgI18NTextFileCharset(textFileCharset);
+      sigEncoding.Assign(textFileCharset);
+    }
+  }
 
   if (NS_FAILED(ConvertToUnicode(sigEncoding.get(), readBuf, sigData)))
     CopyASCIItoUTF16(readBuf, sigData);
diff --git a/mailnews/compose/src/nsMsgCompose.h b/mailnews/compose/src/nsMsgCompose.h
index 64abdaf55eb5..9c9754f13d91 100644
--- a/mailnews/compose/src/nsMsgCompose.h
+++ b/mailnews/compose/src/nsMsgCompose.h
@@ -97,7 +97,9 @@ private:
 
   PRInt32                       mWhatHolder;
 
-  nsresult                      LoadDataFromFile(nsFileSpec& fSpec, nsString &sigData);
+  nsresult                      LoadDataFromFile(nsFileSpec& fSpec,
+                                                 nsString &sigData,
+                                                 PRBool aAllowUTF8 = PR_TRUE);
 
 /*
   nsresult                      GetCompFields(nsMsgCompFields **aCompFields)