Support syncing for more than just images/video

### Is your feature request related to a problem? Please describe.
I would like to be able to sync my audiobooks from my desktop to my phone.

### Describe the solution you'd like
I would like if I could use custom folder connections to sync folders between my phone and desktop for more than just images/video.

I have a folder on my phone called "audiobooks". I create a folder connection between it and a folder on my desktop called "audiobooks". I want to be able to take an audiobook file/collection-of-files/folder/etc and put it in the desktop folder and have the contents appear on my phone.

I can put stuff in the desktop folder and then go into the nextcloud app to download it from the cloud to my phone but I want automatic syncing. I used to use [FolderSync](https://play.google.com/store/apps/details?id=dk.tacit.android.foldersync.full&hl=en_US) to achieve this functionality but I would prefer to use just the nextcloud app since has better UI and more functionality.