Jul 29 2019
All I can say, as a user, is that the problem starts at around 10MB files.
I don't know if I can do this, but I've changed the status to Ubreak Now since it's been impossible to upload anything larger than 10MB since about a week!
Jul 28 2019
If it can be useful: a 10240 KB jpg fill will upload but a 10269 KB jpg file will not.
It looks like the "Internal error: Server failed to store temporary file." message starts at around 10269KB
Jul 25 2019
On page https://commons.wikimedia.org/wiki/Special:UploadWizard
when I upload a 16MB JPG (I tried more than one file) I get "Internal error: Server failed to store temporary file."
It's been happening for the last 24 hours.