Urlencode file name before passing it to cURL

Large file helper use cURL to determine file sizes. Thus filenames must be
urlencoded in case special symbols like '#' can cause BadRequest errors.

Signed-off-by: Tony Zelenoff <antonz@parallels.com>

Backport of 2d03019c91ab8d07dfbfb1e54db86d0c4978ab9a from master