We tested out file transfers last week by having about 15 users send the same Word (.docx) document to two users.
Everyone has identical firewall settings and network connectivity in this test.
One recipient user (me) was set to "available", and the other recipient was set to "Busy". For me, all of the file transfers worked properly.
For the user marked as busy, the transfer would fail after clicking Accept: "You cannot receive the file "Andes Events.docx" from Linda Goscicki.". No prior conversation existed, the senders would open the window and immediately click Send File. After the fail, we would send back an IM, something like "could you try again?" After immediately trying again (by clicking the Send File button a second time), the transfer was successful.
We were able to reproduce the behavior several times. Essentially, for some users, it appears (in our limited testing) that you may need to initiate a text conversation before sending a file for it to work.