Pushing text files result in a wrong mime type error

**Describe the bug**

Using the following curl command :

```
curl -k -H 'Host: synapse.dev.local' -X POST --header 'Authorization: Bearer {user_access_token}' --header 'Content-Type: text/plain' --data-binary 'https://localhost/_matrix/media/v3/upload
```

When getting the file, Matrix content scanner returns the following error :

```
2023-09-27 16:21:04,922 - matrix_content_scanner.scanner.scanner - 518 - ERROR - GET-1348 - Mismatching MIME type (text/plain) and Content-Type header (text/plain; charset=UTF-8)
```