Using url_file_attachment user can specify if he like to change objects
like 'file_share' marked as external (in Slack servers point of view)
to attachments, so the only value would be remembered URL for the
"uploaded" document as an attachment. Or, treat it as is, and let the
slack-backup to produce file which contain the URLs and corresponding
local file names for such files.
In case of user replacement there was a bunch of regular expressions
used, which was an overkill. Substituted them with one generic regexp.
Moved method which is responsible for substitution to the base class.
Last change is a cosmetic one - uploaded files are represented as
absolute path with file:// prefix, so that it is easier to open it with
xdg-open for example.
