Preview API: provide hint for which files (not) to request a thumbnail

## Is your feature request related to a problem? Please describe.
Clients targeting the Preview API have no way of telling in advance for which file types a preview can likely be expected to be available, making (repeated) requests for thumbnails that will never be returned.

## Describe the solution you'd like
Ideally, a list of MIME types, for which the Preview API is likely capable of providing a preview, would be available as part of the capabilities endpoint.

Using that list, clients could then see if the MIME type of an item is in that list and only request thumbnails for those items for which this is the case.

## Describe alternatives you've considered
Alternatively, whether or not a thumbnail is available for a file could be returned as a new WebDAV tag (f.ex. ``).

This approach is more flexible, f.ex. if creating a thumbnail for a file previously failed, the server could tell the client that no thumbnail is available - even though it's a "supported" format.

However, for this additional flexibility to be put to use, I imagine that the server implementation would need to be more complex and possibly lead to a much higher overhead than the proposed solution.

## Related issues
https://github.com/owncloud/core/issues/31267