I’m using Discourse 2.5.0.beta4 and Firefox 76 on Linux. (I do not have a “desktop” on Linux and have no where I can “drag” files from for “drag and drop”.)
I pick a file, and click the “open” button in the file picker.
The image uploads.
The emoji is added to the forum.
I go to add a second one, repeating that same process from step 1 on that same page. Except it never uploads, seemingly returning me to the state just before step 2.
If I use the browser reload button the emoji customization page resets and then I can upload another one.