Comment 19 for bug 704265

I think I figure this out and fixed it in:
https://github.com/jpobst/Pinta/commit/693563bcc7f0fc4e6a29aad1773646c3d3fee669

The empty box was caused by setting the FileChooserDialog filename to "blah.jpg2" when "jpg2" wasn't a valid filter.

Now, if we don't have a filter, we change the extension to whatever is our default filter.

Thanks for the report and continuing to test and provide more info!