Allow attachment of files from Files App to card

[beccon4]

For the time being we can attach files to a card by uploading them from the client's file system.

It would be really nice to be able to attach them from NC's File Application too i.e. attach files already on the cloud.

A current workaround is to share them via link and put it in the description - but for that we have to share the file first which is not always appropriate.

Downloading the file and attach then is problematic as it a) leaves traces on the client side b) is redundant. Linking would allow us to use e.g. Libre Office Online files directly.

[Aquariu]

Dear all, I have created the corresponding Issue on the very aplha Deck Android app. The level of integration we could get by having this looks fantastic!

[juliusknorr]

Duplicate of #564