I am trying to configure nextcloud to allow for uploading some specific file extensions (not all of them), but I am not able to find anything for this.
I saw this that confirm that the rule “File is uploaded” in file access control is not available: Flow: files uploaded in specific folder
I am also looking for a setting to let users upload in a specific folder and not in others, like in their home folder.
Have you seen the app File access control
If this not works you can set the user quota to 0 B.
And then give the user access to Group folders.
Also the app Guests is really cool.
But is it a problem that users save some data in your Nextcloud?
You can set the quota e.g. to only 10 MB or 100 MB.
Perhaps you can explain more about your Nextcloud, the funkctions and your goals.
Yes but I have as default the block access of files, not a filter like block upload of files.
This one seems the best solution indeed.
Probably with this I resolve the issue of uploading in other folders but I am still missing a way to block some file types…
Blocking some file types makes mostly no sense. Whitelists would be the better way. I think that is acually no option in Nextcloud.
For special requirements i use a version adapted by me from Tiny File Manager (upload/rename only for allowed file-suffixes).
$allowed_file_extensions = '';
$allowed_upload_extensions = '';
But on the other hand I have deactivated the user management there 
$use_auth = false;
To me is fine also this solution but I don’t find anything for this and is very strange for a software like this…
I only know this documentation.
https://docs.nextcloud.com/server/latest/admin_manual/file_workflows/access_control.html
Me too but is not clear as we are talking about access how involves upload as it means that the file is already there.
Also there isn’t the picker for filter type File to upload…