added webp to the list of image formats recognized by the files page

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

frontend/src/routes/files/Index.svelte

@@ -6,7 +6,7 @@
     import { t }             from '../../translations.svelte';
     import { user }          from '../../user.svelte';
 
-    const image_extensions = ['jpg','jpeg','gif','png','svg'];
+    const image_extensions = ['jpg','jpeg','gif','png','svg','webp'];
     const router = useTinyRouter();
     let children = $state({});
     let new_dir = $state(null);