Thumbnails don't work with SVG

*This issue was originally created by [**@Dreamsorcerer**](https://github.com/Dreamsorcerer) at .*

### Background information
- **Dendrite version or git SHA**: 0.13.7
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: No
- **`go version`**: 1.21
- **Client used (if applicable)**: Element Web

### Description

- **What** is the problem: Uploaded an SVG as a Spaces icon. No icon appears in UI.
- **How** is this bug manifesting: Error log says `"Failed to read src file" MediaID=3e59ec584581583cdc99db373f07ced46aeadb9bc0388b6abd16fdfaa439cf2a error="image: unknown format" req.id=KzRl2SSkLDWT req.method=GET req.path=/_matrix/media/v3/thumbnail/.../3e59ec584581583cdc99db373f07ced46aeadb9bc0388b6abd16fdfaa439cf2a src=/var/www/matrix/media_store/w/Y/F5VC6SeTYdF4PdcmE21ih6xLVVu7Q4LJx0HvqrzGE/file`

### Steps to reproduce

- Upload icon for space: https://upload.wikimedia.org/wikipedia/en/4/43/Feed-icon.svg
- View that no icon appears in sidebar, spaces page or in settings for spaces page.