#13 Overwrite handling with suffix

[tinco]

When user selects existing file name for export, we now suppress appending the default suffix, even if the name did not appear in the filtered list, so if a user literally types "x" and a file "x" does exist, we will overwrite (after confirm), whereas if "x" does not exist we extend to (and check for overwrite) e.g. "x.png". Is that the best way to handle it?