Hi,
I have just made a test, and I have successfully uploaded (and deleted) a 4.3 MB JPG attachment to this post, on my slow ADSL connection I needed to wait approx. 30s for the upload to be concluded. Maybe that specific file you're trying to upload has a header which the forum has trouble recognizing, and it fails to exit the uploading stage. Could you please upload it somewhere else and provide a link to it here, so that I try to replicate the problem?
Varti