[Feature] Allow adding multiple custom images

**Packages**

Media

**Describe the solution you'd like**

On the custom media page you can currently only upload 1 piece of media at a time and it is immediately previewed.

Instead we could allow uploading multiple pieces and then they all show up underneath similar to how publication media is displayed. The description would just be the file name. We could probably show previews for the images too, videos unlikely but just a generic placeholder would be fine for those, the file name should be sufficient.

**Describe alternatives you've considered**

Current Experience

**Additional context**

N/A

Start at the custom media page and trace how a single upload is handled and immediately previewed, then compare it with how publication media is displayed. Done means multiple images or videos can be selected, each appears underneath with an appropriate preview or generic placeholder, and each uses its file name as the description.

Written by the indexing model from the issue text.