Use correct icon for dir-external-root

When an external storage mount is shared with circles, it triggers the
share icon state to be rerendered. The picking of the mime type icon
would use the regular file icon because there is no actual icon for
"dir-external-root" that is shared.

This fixes the logic to use the "folder-external" icon in such
scenarios.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>