Bug 1362318 - Part2: Fallback override MIME type to application/octet-stream, r=baku

Set override MIME type to 'application/octet-stream', if type is not a parsable
MIME type.

dom/xhr/XMLHttpRequestMainThread.cpp

@@ -1972,6 +1972,13 @@ XMLHttpRequestMainThread::OnStartRequest(nsIRequest *request, nsISupports *ctxt)
     channel->SetContentType(NS_ConvertUTF16toUTF8(mOverrideMimeType));
   }
 
+  // Fallback to 'application/octet-stream'
+  nsAutoCString type;
+  channel->GetContentType(type);
+  if (type.Equals(UNKNOWN_CONTENT_TYPE)) {
+    channel->SetContentType(NS_LITERAL_CSTRING(APPLICATION_OCTET_STREAM));
+  }
+
   DetectCharset();
 
   // Set up arraybuffer

testing/web-platform/meta/XMLHttpRequest/overridemimetype-blob.html.ini

@@ -1,8 +1,5 @@
 [overridemimetype-blob.html]
   type: testharness
-  [Bogus MIME type should end up as application/octet-stream]
-    expected: FAIL
-
   [Bogus MIME type should end up as application/octet-stream, 2]
     expected: FAIL