Use page title for Content-disposition filename

Pywikibot has sent the local filename as the Content-disposition filename
since it was first implemented in Pywikibot 2.0:
    f08b87dd56d4096e82eea6b6f0c35e260b2038f2

This is unnecessary, and leaks the local path of the file.
Pywikipedia sends the filename returned by process_filename(),
which doesnt include the path.

Change-Id: Ib751ee3f4074a60f3b53b0afe3cc2dfc3e17b2f7
Merge "Use page title for Content-disposition filename"