Remove the filename argument from Content-Disposition header to avoid

RFC 5987 and RFC 6266 encoding issues. This change allows the browser
to determine the filename based on the URL.

See: http://greenbytes.de/tech/tc2231/

Closes https://github.com/gitlabhq/gitlabhq/issues/9595

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/1829