Deal with missing filename extensions for video files.

sync.js

@@ -127,7 +127,11 @@ async function doWork() {
         const parts    = post.media_url.split('/');
         // HACK: despite the .heic extensions, these are JPEG files right now
         // This will probably change in the future and break things.
-        const fileName = parts[parts.length - 1].split('?')[0].replace('.heic', '.jpg');
+        var fileName = parts[parts.length - 1].split('?')[0].replace('.heic', '.jpg');
+        // as of June 2024, no extension seems to mean "video"
+        if (fileName.indexOf('.') == -1) {
+            fileName += '.mp4';
+        }
 
         const downloader = new Downloader({     
           url: post.media_url,