Extend configuration option to prompt for synchronisation of large folder to also include an option (separate or not) for large files.

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### Feature description
As outlined in #4206, it's currently possible to add large files to existing folders (whether empty or not) without getting prompted that you are about to sync huge files. This is confusing for users if for example someone working at a media company adds a large video project to an existing shared folder. All others would automatically get those files downloaded and would be unaware why they suddenly lost 10GB or more.

While I would love to see #4206 fixed, I wonder if a folder based approach is the most user-friendly option here. I would argue that cases where suddenly a huge file (e.g. a video) is added are more common than a folder containing 500 files of 1MB. I would therefore suggest that either the setting is recontextualised to apply both to folders and individual files, or that a separate setting is added to ask for confirmation before synchronizing files larger than a certain size.

I must admit I was unsure whether I should file this as an enhancement or bug. While it is true that this is a request to add an extra feature, it's also the case that the currently behavior is confusing to users, and therefore I would classify it in a sense as a UX bug.