Unique filenames for downloaded images

### Your use case

When other users send images from their clipboards (or by drag and drop), the files are given generic names such as `image.jpg` or `image.png`. This presents a problem for the receiving user(s) when downloading the image within Element, as the user is prompted to either overwrite an existing file with the same name, or give a unique name to the image.

This can be quite tedious, and effectively defeats the benefit of the easy-to-access download button next to each message, since the user has to stop and give a unique filename to each downloaded image.

I propose having Element automatically name files based on their Matrix Content IDs (`mxc://`) when downloaded, if they would otherwise have an `image.*` filename.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No file or test is named in the issue. Start by tracing the image download button and the code that assigns fallback image.* filenames; done means downloads that would otherwise use a generic image name receive a unique name based on the Matrix mxc:// content ID.

Written by the indexing model from the issue text.