[fixed] No-extension filename failed to cooperation with renderer.

[micheus]

In the state of unsaved Wings data, export file name is blank.
If a file name is blank,an user may save the file without the extension.
:
:
Suggestion:
I think that necessary to devise for the user does not make mistakes.
For example,when unsaved state, to autofill the filename "Untitled" and extensions in export dialog.

tkbd, thanks for the report and suggestion.
I fixed it to work that way.