When attempting to upload a pre-existing map to a Realm, the game will either crash you back to the dashboard or will hang on the "Initializing Upload" screen (while also reporting the maps size as 0.00MB) until a failure message is given. 

I have attempted this with a few maps. Out of my testing, superflat/maps with small file sizes can successfully be uploaded to a Realm, but larger maps cannot (62.2MB in my case).

EDIT: When the game gets stuck on the "Initializing Upload" screen, my Internet does get very slow, so the game is doing something. Eventually it just times out (it definitely sits on the screen for too long. My internet is slow, but 60MB should be uploaded in less than 5 minutes) and gives me an error window ingame with no text in the box, but "Failed" as the title.