For missing/invalid extensions, append the correct extension if we know it.

4chan will reject WebMs sent as ".jpg".

src/Posting/QR.coffee

@@ -12,6 +12,15 @@ QR =
     'swf':  'application/vnd.adobe.flash.movie'
     'webm': 'video/webm'
 
+  extensionFromType:
+    'image/jpeg': 'jpg'
+    'image/png': 'png'
+    'image/gif': 'gif'
+    'application/pdf': 'pdf'
+    'application/vnd.adobe.flash.movie': 'swf'
+    'application/x-shockwave-flash': 'swf'
+    'video/webm': 'webm'
+
   init: ->
     return unless Conf['Quick Reply']
 

src/Posting/QR.post.coffee

@@ -301,10 +301,8 @@ QR.post = class
   saveFilename: ->
     @file.newName = (@filename or '').replace /[/\\]/g, '-'
     unless QR.validExtension.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'
+      # 4chan will truncate the filename if it has no extension.
+      @file.newName += ".#{QR.extensionFromType[@file.type] or 'jpg'}"
 
   updateFilename: ->
     long = "#{@filename} (#{@filesize})"