It's per file, although I also think it applies per message (i.e., you can upload 10 photos at a time, but they can't collectively exceed 50MB on the free plan; you'd have to split them across multiple messages).
It's 50mb on Nitro Basic, and like 5mb on free tier. This has bit me a few times when sharing screenshots (4K ultrawide), but I still wouldn't pay for a service Imgur & co provides for free.