Fixes the regression introduced by
  commit e637b674

This commit removed the crazy bSubjectIsFilename handling,
where I didn't understood the case of bSubjectIsFilename
and a user supplied prefix.

Mail merge to files never overwrites an existing document,
but there is the special case, when a user selects a target
filename in the MM dialog for single file MM.

Should be fixed by a successive commmit, reverting this and
removing an existing file before starting the MM job.

Change-Id: Idda487023e6984de9c1e701fc088a6b7f92e9847