If you're trying to upload a .jar (Not .rar derp)
You need to compress it with winrar
1) Create a new thread
2) 'Manage Attachments'
3) Add an attachment
4) Click 'Done'
(Make sure you have screenshots and two virusscans in your thread)
And what extension did your file have?
(Only the following extensions can be uploaded using the Attachment System: bmp doc gif jpe jpeg jpg pdf png psd rar txt zip)