Clients targeting the Preview API at remote.php/dav/files/[user]/[file]?x=[width]&y=[height]&c=[eTag]&preview=1 have no way of telling in advance for which file types a preview can likely be expected to be available.
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.
Clients targeting the Preview API at
remote.php/dav/files/[user]/[file]?x=[width]&y=[height]&c=[eTag]&preview=1have no way of telling in advance for which file types a preview can likely be expected to be available.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.