For anyone reading this later, it is a little easier now. Goal is: every file that gets created/moved into a certain folder should receive a tag.
You have to give the folder a tag, let’s say YOURTAG. Then in adminsettings > Flow > create a new flow for automated files tagging:
Rule 1
file is tagged with -> YOURTAG
Rule 2
file mime-type -> IS NOT -> folder
Automatically tag with:
Choose another Tag, let’s say MYFILETAG.
Now only the files from inside the folder you tagged with YOURTAG have the MYFILETAG tag. Voila.