fix(PublicPreview): Call getMimetype() on File objects

Signed-off-by: Git'Fellow <12234510+solracsf@users.noreply.github.com>

Author: solracsf

apps/files_sharing/lib/Controller/PublicPreviewController.php

@@ -132,7 +132,7 @@ public function getPreview(
 			return $response;
 		} catch (NotFoundException $e) {
 			// If we have no preview enabled, we can redirect to the mime icon if any
-			if ($mimeFallback) {
+			if ($file instanceof \OCP\Files\File && $mimeFallback) {
 				if ($url = $this->mimeIconProvider->getMimeIconUrl($file->getMimeType())) {
 					return new RedirectResponse($url);
 				}