It seems it just won't work when uploading it via the attachment system (even though the file size doesn't go over the limit.
I don't have access to TiberiumWeb's FTP, but I can upload the map to DTA's FTP. I'll add a link to it to it as soon as it finishes uploading.
Edit: it now seems there might be something wrong with tibweb's connection... While trying to uplaod the map to the FTP, the transfer keeps getting interrupted, because of which it can't finish (which explains why the attachment wouldn't upload earlier).
Fortunately when uploading to an FTP, you have the option to proceed uploading after reconnecting to the FTP (so I should manage to finish this upload)

Edit2: it it doesn't seem to work after all... After continuing an upload after an interruption, it'll interrupt a while later again and after continuing the interrupted upload a couple times, it starts over again