[Bug]: ffmpegthumbnailer.thumbnailer is missing `video/matroska` MIME type

### Kernel

7.1.3-2-cachyos

### Desktop Environment / Window Manager

Other (specify in additional context)

### CPU

AMD Ryzen 9 7940H (16) @ 5.26 GHz

### GPU / Driver

AMD Radeon 780M Graphics [Integrated]; NVIDIA GeForce RTX 4060 Max-Q / Mobile [Discrete]

### Description

I'm using Niri WM.

The packaged /usr/share/thumbnailers/ffmpegthumbnailer.thumbnailer
contains `video/x-matroska;` `application/x-matroska;` but does not include `video/matroska;`

On my system, `gio info file.mkv` reports:
```
standard::content-type: video/matroska
```
As a result, **Nautilus (GNOME Files) does not generate thumbnails for any MKV files.**

After adding `video/matroska` to the MimeType field, thumbnails are generated correctly.

### Steps to Reproduce

Enter any directories containing MKV files in Nautilus.

### Expected Behavior

Nautilus does not generate thumbnails for any MKV files.

### Actual Behavior

Nautilus should generate thumbnails for all MKV files.

### Logs / Error Messages

None

### Additional Context

_No response_

Inspect the packaged /usr/share/thumbnailers/ffmpegthumbnailer.thumbnailer file and its MimeType field; compare the listed Matroska types with the content type reported by gio info for an MKV file. Done means video/matroska is included and Nautilus generates thumbnails for MKV files.

Written by the indexing model from the issue text.