There is a group permission to allow using attachments. Your id needs to be in a group that allows for attachements to be uploaded.
In addition, there can be limits on which types of files and quotes defined for each group. This might be related.
Finally, some older versions/distros of jForum required that a tmp directory be created in the webapp root directory. Don't think this applies because errors related to this were due to large files... but check it anyway. [originally posted on jforum.net by monroe]