--- a/content/base/src/nsDocument.cpp
+++ b/content/base/src/nsDocument.cpp
@@ -2255,17 +2255,20 @@ nsDocument::StartDocumentLoad(const char
 
   mHaveInputEncoding = true;
 
   if (aReset) {
     Reset(aChannel, aLoadGroup);
   }
 
   nsAutoCString contentType;
-  if (NS_SUCCEEDED(aChannel->GetContentType(contentType))) {
+  nsCOMPtr<nsIPropertyBag2> bag = do_QueryInterface(aChannel);
+  if ((bag && NS_SUCCEEDED(bag->GetPropertyAsACString(
+                NS_LITERAL_STRING("contentType"), contentType))) ||
+      NS_SUCCEEDED(aChannel->GetContentType(contentType))) {
     // XXX this is only necessary for viewsource:
     nsACString::const_iterator start, end, semicolon;
     contentType.BeginReading(start);
     contentType.EndReading(end);
     semicolon = start;
     FindCharInReadable(';', semicolon, end);
     SetContentTypeInternal(Substring(start, semicolon));
   }