Introduce a list of image mime types we can support as images

E.g. TIFF will now be handled as a generic file, not image, even though it has mime prefix "image/"

src/components/messages/composition/useThumbnail.ts

@@ -32,7 +32,7 @@ export const useThumbnail = (source: KeanuEvent | File | undefined) => {
   } else if (source) {
     const file = source as File;
     isVideo.value = file.type.startsWith("video/");
-    isImage.value = file.type.startsWith("image/");
+    isImage.value = utils.isSupportedImageType(file.type);
     fileName.value = file.name;
     fileSize.value = prettyBytes(file.size);
   }