Attachments in E2EE broken in chat export

### Steps to reproduce

1. Export chat in an E2EE room with some images in it
2. Check the 'include attachments' box
3. Try to view the resulting HTML
4. Discover the images don't display, presumably because they're undecrypted AES

### Outcome

#### What did you expect?

Attachments should be decrypted and displayable.

#### What happened instead?

Attachments look to be encrypted. Probably an authenticated media regression.

### Operating system

_No response_

### Application version

Element Nightly version: 2024101001 Crypto version: Rust SDK 0.7.2 (517d99b), Vodozemac 0.7.0

### How did you install the app?

nightly

### Homeserver

matrix.org

### Will you send logs?

No