Markdown images with mxc:// URLs are not displayed

### Steps to reproduce

Whenever somebody sends a message formatted with Markdown, and it includes an image with an mxc:// URL (for example ``), the image is not displayed. The image is correctly displayed in the web client.

To reproduce:
- Upload an image from the web client. This should display correctly in the iOS client and in the web client.
- Click "View source" in the message properties to find out the mxc url. It should look like this: `mxc://matrix.server/eYQIShWKWDWBrfCSpGDPlVmK`
- Send an image from a web client with this content: ``
- It will display correctly in the web client, and will be absent in the iOS client

### Outcome

#### What did you expect?
I expected the image to be visible in all clients.
#### What happened instead?
The iOS client displays an empty message.

### Your phone model

iPhone 11

### Operating system version

iOS 17.2.1

### Application version

1.11.7

### Homeserver

Synapse v1.98.0

### Will you send logs?

No