What version of mIRC are you running, and were you sending between the same client? (or did you open a second mIRC?)
The "file.ext" only exists to maintain backwards compatibility with older clients, as it is redundant. That is, the clients don't have to send the filename back and forth to each other, as the port allows them to identify what file they're talking about (one port per number, per machine, hence the port number is unique to the sendee machine).
I have got this problem once, and when I tried to reproduce it it was gone. If the problem still persists, it might be helpful to paste the full dcc negotiation.
Hope that helps.
