Randomize Filename: Keep original extension as it will be overwritten anyway.

src/Posting/QR.coffee

@@ -1,6 +1,8 @@
 QR =
   mimeTypes: ['image/jpeg', 'image/png', 'image/gif', 'application/pdf', 'application/vnd.adobe.flash.movie', 'application/x-shockwave-flash', 'video/webm']
 
+  validExtension: /\.(jpe?g|png|gif|pdf|swf|webm)$/i
+
   typeFromExtension:
     'jpg':  'image/jpeg'
     'jpeg': 'image/jpeg'
@@ -10,15 +12,6 @@ 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

@@ -179,10 +179,11 @@ QR.post = class
     return
 
   setFile: (@file) ->
-    @filename = if Conf['Randomize Filename'] and g.BOARD.ID isnt 'f'
-      "#{Date.now() - Math.floor(Math.random() * 365 * $.DAY)}.#{QR.extensionFromType[@file.type] or 'jpg'}"
+    if Conf['Randomize Filename'] and g.BOARD.ID isnt 'f'
+      @filename  = "#{Date.now() - Math.floor(Math.random() * 365 * $.DAY)}"
+      @filename += ext[0] if ext = @file.name.match QR.validExtension
     else
-      @file.name
+      @filename  = @file.name
     @filesize = $.bytesToString @file.size
     @checkSize()
     @nodes.label.hidden = false if QR.spoiler
@@ -299,7 +300,7 @@ QR.post = class
 
   saveFilename: ->
     @file.newName = (@filename or '').replace /[/\\]/g, '-'
-    unless /\.(jpe?g|png|gif|pdf|swf|webm)$/i.test @filename
+    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.