If you were to rename the file from 'c1240-k9w7-tar.default' to 'c1240-k9w7.tar' that should fix it; otherwise, if you have access to a Windows or Linux desktop box, check out this program:
tftpgui.py
The windows installer contains a minimal Python interpreter; the Linux box should have one already installed.
Here's the page for it:
http://code.google.com/p/tftpgui/
Click the "Wiki" tab for more information; click the "Show All" link next to "Featured Downloads" to see both Windows installers and the Linux tarfile.
I had to upload a firmware file to an Aironet 1100 and this was the easiest tool I found available for the job.
I tinkered a little bit with the SolarWinds TFTP server, but it didn't accept connections under XP under VirtualBox on my Linux machine.
Hope this helps!
"Merch"