Drag & drop support from macOS Mojave's new screenshot UI

Since macOS Mojave, a new screenshot UI was added. This UI shows us a thumbnail of the screenshot we have taken in the lower right corner of the screen, without directly creating a new file in the desktop.

The usual workflow with this new UI is to take an screenshot and drag the thumbnail to any place you want to paste it into. However, the Discord app currently does not support this functionality; it completely ignores the fact that you're trying to drop a file to send it in a channel.

My suggestion is simple, to support this functionality like other chat applications currently do.