Extension-less exports always become WAVs

### System Information

Linux core2 6.18.2-arch2-1 #1 SMP PREEMPT_DYNAMIC Thu, 18 Dec 2025 18:00:18 +0000 x86_64 GNU/Linux

### LMMS Version(s)

LMMS 1.3.0-alpha.1.950

### Most Recent Working Version

_No response_

### Bug Summary

When exporting, typing in an extension-less filename always results in a WAV export regardless of the selected filetype.

### Expected Behaviour

It should follow the selected filetype.

### Steps To Reproduce

1. Press `Ctrl+E` or go to `File > Export...`
2. Type an extension-less filename on the dialog that appears, and select a file format that isn't `wav`.
3. Click `save`
4. The export dialog filetype will go to `wav`.

### Logs

_No response_

### Screenshots / Minimum Reproducible Project

### Please search the issue tracker for existing bug reports before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by reproducing the issue through File > Export or Ctrl+E on Linux, using an extension-less filename and a non-WAV format. Trace the export dialog's filename and filetype handling to find why saving resets the selection to WAV. Done means an extension-less export preserves the selected filetype and the reproduction steps no longer show WAV unexpectedly.

Written by the indexing model from the issue text.