No thumbnails when sending image

# Steps to reproduce

1. Take an image and send it
2. (Have good internet so that sending does not fail)
3. Long tap on the message
4. View decrypted source

The event does not include any thumbnail(s), even though it should. High quality images may take more than 10 MB in size, so it is essential to have a small (<100 KB) preview version to show.

# Environment

- Element version: `1.0.0 [40100000]`
- Matrix SDK Version: `0.0.1`
- Olm version: `3.1.2`
- Encryped room