File extension isn't adjusted when the file name contains an extension and the format is changed in the export settings dialogue.

### Bug Summary
Affected system: Windows 8.1 64 bit

#### Steps to reproduce
Start LMMS.
Make a track in the song editor.
Select "Export", select one of the formats in "Files of type" in the file selection dialogue, and continue to the next dialogue without erasing the extension in the file name.
Change the file to a different format in the export settings dialogue.
Export the project.

#### Expected behavior
The output file should have an extension (either changed or appended) that corresponds to the format selected in the export settings dialogue.

#### Actual behavior
The output file retains the extension from the first dialogue despite being encoded in a different format.

#### Affected LMMS versions

Version 1.2.2 (win32/x86_64, Qt 5.4.2, GCC 4.9.4 20150716 (prerelease))