Allow posting videos

[schwa]

There are now two code paths for adding media attachments:

1. Drag and drop.
2. PhotoPicker

Adding video with D&D is relatively easy. The PhotoPicker stuff doesn't seem to work well. We can get Data() back from it but we don't know the content type. For image with can use ImageSource and sniff it. No such function exists for Video?

We can possibly refactor all this and make both D&D and PhotoPicker work the same way. Much refactoring needed.