Perk,
Most probably your file size is too big for an attachment (afair it's 512 kb now).
You can upload bigger file to our Kelly's server, see information here:
http://www.sketchucation.com/forums/scf/viewtopic.php?f=77&t=760
or actually, if the file is not a "secret" one, even to the 3D WareHouse (you can upload 10 mb files there).