I've just checked with our database. You're allowed to post attachments. The problem might be with the browser you're using, connection troubles or you ISP. According to our data you must have no problem posting attachments. Could you check with other possible sources of problems apart from GMAT Club?
Sorry for inconveniences.
Welcome to GMAT Club!
Want to solve GMAT questions on the go? GMAT Club iPhone app will help.
Please read this before posting in GMAT Club Tests forum
Result correlation between real GMAT and GMAT Club Tests
Are GMAT Club Test sets ordered in any way?
Take 15 free tests with questions from GMAT Club, Knewton, Manhattan GMAT, and Veritas.
GMAT Club Premium Membership - big benefits and savings