git-remote-mediawiki: send "File:" attachments to a remote wiki

git-remote-mediawiki: send "File:" attachments to a remote wiki

The current version of the git-remote-mediawiki supports only import and
export of plain wiki pages. This patch adds the functionality to export
file attachments (i.e. the content of the File: MediaWiki namespace),
which are also exposed by MediaWiki API.

This requires a recent version of MediaWiki::API (Version 0.37 works.
Version 0.34 doesn't).

Signed-off-by: Pavel Volek <Pavel.Volek@ensimag.imag.fr>
Signed-off-by: NGUYEN Kim Thuat <Kim-Thuat.Nguyen@ensimag.imag.fr>
Signed-off-by: ROUCHER IGLESIAS Javier <roucherj@ensimag.imag.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>