Animated GIFs (or WEBPs) don't play in maximized view

### Steps to reproduce

1. Select an animated GIF (or WEBP) file to upload to a chat room
2. The GIF is not animated in the "Add a caption" screen
3. The GIF is animated in the chat view
4. Touch the GIF to view it in full screen and it is not animated

### Outcome

The animated GIF (or animated WEBP) should be animated in the fullscreen view but it is not

### Your phone model

Pixel 6 Pro

### Operating system version

Android 15

### Application version and app store

25.05.3

### Homeserver

Synapse 1.128.0

### Will you send logs?

No

### Are you willing to provide a PR?

No

Start by reproducing the issue with an animated GIF or WEBP through the Add a caption screen, chat view, and fullscreen view on Android. Trace the fullscreen media viewer and compare its handling with the chat view; done means both animated formats remain animated in fullscreen.

Written by the indexing model from the issue text.