Do not forget to share all the data, not the metadata. For example, a .vtm or .pvd file would not be enough, all the associated dataset should be shared as well.
Here is the resolution to my problems: The limit is 10 MB, not 20 MB.
I tried many times to upload my 15 MB file. I tried tar.gz and I tried zip format. The upload indicator would go to 100% and then fail for an unspecified reason (it would be nice if you could tell users why the upload fails).
According to Google AI, “The default upload limit for many Discourse instances is 10 MB.” That was the key.