Wikimedia Bugzilla is closed!

if you don't (or forget to) attach a file extension to the destination name
mediawiki puts out the following error:
"." is not a recommended image file format.

that is a bit confusing, it should say something like this:
you did not attach a file extension to the destination name, please enter any to
upload your file.

ps: maybe you could check what file extension the selected file has and attach
it to the destination name?