File base name is no changed after format changing in "Export Image" dialog

Scenario to reproduce:

1. Open "data/samples/CLUSTAL/COI.aln".
2. Export an msa as image.
   Expected state: "Export Image" dialog is shown. A result file base name is "untitled" or "untitled_copy" with a number.
3. Change format to any non-default and export the msa.
4. Call the dialog again.
   Expected state: "Export Image" dialog is shown. A result file base name is the same as in the previous time.
5. Change format to the same as in the previous time.
   Current state: the result file base name is not changed, despite of file with the same name already exists.

The file base name should be changed to avoid rewriting user's data.