The image had been stored as "data:image/...;base64;<data>", which
the context menu Save Image and Copy Image could not handle. This
resulted in a lengthy error message, which could cause a crash.
This commit fixes several things:
a) replace "data:" URI-s with "mail://" URI-s, which means less resources
   are needed; it also fixes the Save/Copy Image actions as a side effect;
b) fix memory leak for tiff images;
c) let to Save/Copy Image handle base64-encoded "data:" URI-s (as
   the source HTML message can still contain them);
d) shorten the URI in the error message when it's too long;
e) pass also "filename" into the URI, thus the Save Image has prefilled it,
   the same as attachment's "Save as" action.

Reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1657361