diff --git a/mailnews/compose/src/nsMsgAttachmentHandler.cpp b/mailnews/compose/src/nsMsgAttachmentHandler.cpp
index 0ae253ed6e..9555de37fc 100644
--- a/mailnews/compose/src/nsMsgAttachmentHandler.cpp
+++ b/mailnews/compose/src/nsMsgAttachmentHandler.cpp
@@ -522,7 +522,9 @@ DONE:
 nsresult
 nsMsgAttachmentHandler::PickCharset()
 {
-  if (!m_charset.IsEmpty() || !m_type.LowerCaseEqualsLiteral(TEXT_PLAIN))
+  if (!m_charset.IsEmpty() ||
+      !StringBeginsWith(m_type, NS_LITERAL_CSTRING("text/"),
+                        nsCaseInsensitiveCStringComparator()))
     return NS_OK;
 
   nsCOMPtr<nsIFile> tmpFile =