Update MongoDB connector

lib/fs.js

@@ -27,19 +27,27 @@ function listener(request, response) {
   const urlParts = url.parse(request.url, true);
   if (request.method === "GET") {
     const filename = querystring.unescape(urlParts.pathname.substring(1));
-    const gs = new mongodb.GridStore(db.mongoDb, filename, "r", {});
-    gs.open(err => {
+    db.filesCollection.findOne({ _id: filename }, (err, file) => {
       if (err) {
+        response.writeHead(500, { Connection: "close" });
+        response.end(`${err.name}: ${err.message}`);
+        throw err;
+      }
+
+      if (!file) {
         response.writeHead(404);
         response.end();
         return;
       }
-      const stream = gs.stream(true);
+
       response.writeHead(200, {
-        "Content-Type": "application/octet-stream",
-        "Content-Length": gs.length
+        "Content-Type": file.contentType || "application/octet-stream",
+        "Content-Length": file.length
       });
-      stream.pipe(response);
+
+      const bucket = new mongodb.GridFSBucket(db.client.db());
+      const downloadStream = bucket.openDownloadStream(filename);
+      downloadStream.pipe(response);
     });
   } else {
     response.writeHead(405, { Allow: "GET" });