The last major spam attack a few months back has caused posts to be moderated.
I am going to guess that some rules have been setup that will flag posts for moderation if there are some links that can not be verified without some human intervention. Might be especially true for new members without a track record.
Can the files be posted directly to the forum? I think the limit for zipped files is 20-25 megs.
once they get through the moderation process, uploading files doesn't seem to be an issue.