Unless I’m unaware of something, any files I put into my Nextcloud files is copied over to my phone. I would rather be able to determine which files are downloaded to my phone. I installed the Nextcloud client which shows me the files on the web and doesn’t download them by default to my phone, which is preferable.
To prevent sync of all data you can go to Settings >>Accounts >>Select you /e/ ID >> Account sync - in the screen that comes up disable items which you do not want to sync. By default all items in the list are synced. You can raise an issue in Gitlab to prevent auto sync by default and let user choose what he / she wants to sync.
I don’t see anything there for files. Could it be “Application Settings”? No, that didn’t seem to do anything with the files, although it does beg the question: What is being synced under “Application Settings”?
Also, it’s odd that under Accounts there’s one for my /e/ account, which includes contacts under there, but then there’s also a Contacts account with the same /e/ account. Finally there is a NextCloud account for my /e/ account, but it has File sync off.