Add a fake filename extension when needed.

src/Posting/QR.coffee

@@ -554,6 +554,11 @@ QR =
         when 'filename'
           return unless @file
           @file.newName = @filename
+          unless /\.(jpe?g|png|gif|pdf|sfw)$/i.test @filename
+            # 4chan will truncate the filename if it has no extension,
+            # but it will always replace the extension by the correct one,
+            # so we suffix it with '.jpg' when needed.
+            @file.newName += '.jpg'
           @updateFilename()
     forceSave: ->
       return unless @ is QR.selected