Attachments are stored on the forum server. Legally, that's risky when it comes to hosting copyrighted data (such as server files) via attachments. Specially knowing the purpose of the forum, that's likely happening (already). Links refer to data stored some place else. I'm not a lawyer but I guess it's much less trouble for RZ if copyright infringing data is stored outside of the RZ servers. So keeping the upload limit for attachments fairly low forces people to upload their stuff somewhere else.