Frontend: don't show img preview for video/audio/doc files

- MediaPreview.vue: only render <img> for non-video, non-audio items
  (video/audio already have their own player containers)
- Document.ts: limit previewurl to previewable files (was checking only .complete)

frontend/src/components/MediaPreview.vue

@@ -2,7 +2,7 @@
   <div v-if=showProgress() class="preview-progress" aria-label="Preview pending">
     <SpinnerIcon />
   </div>
-  <img v-else-if=previewSrc :src="previewSrc" alt="">
+  <img v-else-if="previewSrc && !video() && !audio()" :src="previewSrc" alt="">
   <img v-else-if=doc.img :src=doc.url alt="">
   <span v-else-if=doc.dir class="folder icon"></span>
   <div v-else-if=video() class="video-container" :class="{ pending: !doc.complete }">