Somewhere online there should be a list of the e-filing rules and forms allowed. For example, in past years, you could only attach 4 copies of form 8283 (for non-cash charity donations) to your return. If you had so many non-cash donations that you needed more than 4 of that form, you had to mail your return.
Unfortunately, I don't know where that list is for this year, and I didn't find it in a quick search. A tax professional might know.
