Hi,
Thank you for the honest reply ! 
Knowing that the file is indeed copied into the RAM, I can safely say it’s “just” a memory issue on my part. I actually feel better knowing that, I was afraid it would be a major memory leak bug. I don’t have enough RAM, eh, I can live with it.
If it could be improved, it would most be welcome, but I understand the necessity to manage uploads that way, so if I’m the only one who complained about it, consider it low priority. (Maybe “if the file is not a multimedia file we need to analyze, write directly to disk” as an option or something ?)
But I do think it would be nice if it could be detailed in the doc, I don’t remember reading it somewhere.
In the meantime, we did just what you recommended. I set up a Seafile server next to the Mattermost, so large files can be uploaded there.
Thanks again for the reply.