make sure the path is set correctly as to where you are uploading the files to. You can locate it at:
Your Boards » Control Panel » Feature Settings - Attachments
Make sure you set how large a file can be and also make sure the permissions are set on that directory so you can write files to it. These are the basics..
Also there is upload functionality in your php.ini file on the server. You may want to check that for any size restraints set at the php level.
This is surface stuff I can think of.
