Looks like a fix for 3.4.3.40990 does not work as intended.
“Fix errors when downloading files with no extension or MIME type”
Duplicates are created on the cloud (e.g. “DESCRIPTION” file gets uploaded and is renamed to “DESCRIPTION 1” on the cloud)
Version: 3.4.3.40990
One-Drive for business
This does not affect all files without extension though.