Don't use --mime-type

The `file` command on OS X takes the --mime-type option, which returns
only the type and not the optional encoding. This is nice, but it's not
portable. Use --mime and strip the encoding off if it's there.

And don't forget this time.

lib/paperclip/media_type_spoof_detector.rb

@@ -52,7 +52,7 @@ module Paperclip
 
     def type_from_file_command
       begin
-        Paperclip.run("file", "-b --mime-type :file", :file => @file.path)
+        Paperclip.run("file", "-b --mime :file", :file => @file.path).split(/[:;]\s+/).first
       rescue Cocaine::CommandLineError
         ""
       end