yes, that's good idea to disallow unneeded files and save traffic for some users
Even better would be to (also) allow users to enter regular expressions to specify exactly what kind of files they want to skip. I'm using this excessively in Nicotine+ to remove all the duplicate album art and such, while still being able to download jpg files.
I've added file extension filtering to my todo list. Hopefully I can get it done by the next build.