FileOpen dialog confusing

### Problem description

FIleopen dialog is showing *.FCstd and *.FCbak files which can be very confusing when looking for a file to open, as below, especially if you keep more than one backup copy.

Is it possible to get back to just showing the *.FCstd files.
This happened with commit 'Enable direct opening of .FCBak backup files without renaming
#28454' 1st April 2026.
Maybe we need to change the 'Suggested Formats' criteria from *.FCstd and *.FCbak to select only *.FCstd or only *.FCbak

### Workbench affected?

None

### Steps to reproduce

Click 'open file' and see what a mess it looks.

### Expected behavior

See *.FCstd files only

### Actual behavior

FCstd and FCbak files are shown

### Development version About Info (in Safe Mode)

```shell
OS: Windows 10 build 19045
Architecture: x86_64
Version: 1.2.0dev.20260505 (Git shallow)
Build date: 2026/05/05 22:34:03
Build type: Release
Branch: (HEAD detached at 08cacbbc2)
Hash: 08cacbbc27421e769be4ec423f9815a01e51e533
Python 3.11.15, Qt 6.8.3, Coin 4.0.8, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.2, OCC 7.8.1
Locale: English/United Kingdom (en_GB) [ OS: English/United Kingdom (en_GB) ]
Stylesheet/Theme/QtStyle: My-FreeCAD.qss/My-FreeCAD/
Navigation Style/Orbit Style/Rotation Mode: Gesture/Trackball/Object center
Logical DPI/Physical DPI/Pixel Ratio: 96/102.299/1
```

### Last known good version (optional)

```shell
OS: Windows 10 build 19045
Architecture: x86_64
Version: 1.2.0dev.20260324 (Git shallow)
Build date: 2026/03/24 17:14:33
Build type: Release
Branch: (HEAD detached at 1bc5980a7)
Hash: 1bc5980a7ff302a55c617181e774cd5cd2a9443a
Python 3.11.15, Qt 6.8.3, Coin 4.0.8, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.2, OCC 7.8.1
Locale: English/United Kingdom (en_GB)
Stylesheet/Theme/QtStyle: My-FreeCAD.qss/My-FreeCAD/
Navigation Style/Orbit Style/Rotation Mode: Gesture/Trackball/Object center
Logical DPI/Physical DPI/Pixel Ratio: 96/102.299/1
Installed mods:
* A2plus 0.4.68
* Addon Manager 2026.4.25
* Assembly4 0.60.6
* My-Darker-Flat
* My-Flat
* My-FreeCAD
```

Start by reviewing commit #28454 and the file-dialog filtering changes that enabled direct opening of .FCbak files. Reproduce the Open File dialog on Windows and compare the suggested formats with the expected .FCstd-only view. Done means .FCstd files are shown without the backup files making the list confusing, while the reported backup-opening behavior remains considered.

Written by the indexing model from the issue text.