Hi Leire,
it seems your file uploader is not working. Probably, you also cannot upload files to your "Private files" area, and cannot add a "File" resource either.
I suggest you try first purging the Moodle cache (Site administration -> Development -> Purge all caches), and then clear your browser cache of all temporary files.
Some other things you could experiment with are disabling JavaScript on your browser, and also disabling AJAX on your Moodle site.
Please let us know if you find a solution.
regards
Gordon