Most images don't load using 0.13.8

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

### Background information

- **Dendrite version or git SHA**: 0.13.8+763c79f
- **SQLite3 or Postgres?**: Postgres (15-alpine)
- **Running in Docker?**: Yes
- **`go version`**: `sh: go: not found`
- **Client used (if applicable)**: Element on Web, Element X, Cinny

### Description

- **What** is the problem: Requested images return no data, but status 200 OK
- **Who** is affected: Single user
- **How** is this bug manifesting: Images rarely or never load in when I'm using the above clients
- **When** did this first appear: Since the 0.13.8 update

I've tried resetting all my clients too. Example below returns `Content-Length: 144` with `Content-Type: application/json` without data. Couldn't find any relevant logs either.

### Steps to reproduce

- Setup Dendrite monolith using Docker
- Use any of the above clients

See images below





Start by reproducing the issue with a Docker monolith, Postgres 15, and one of the listed clients, then inspect the response to an image request. The fix is complete when requested images return image data with the expected content type instead of an empty JSON response with status 200.

Written by the indexing model from the issue text.