Bump nodejs version to 6+ (#4272)

* let travis build against 3.x

* Cleanup dependencies and bump min version to current LTS

* Makes npm-git push all branches to -preview

* restores releases

* Bumps mime to 2.0.3 (requires node 6+)

* Bumps express to latest version

* Fixes linting issue after upgrade

* Use travis-branch for partial releases

src/Routers/FilesRouter.js

@@ -37,7 +37,7 @@ export class FilesRouter {
     const config = Config.get(req.params.appId);
     const filesController = config.filesController;
     const filename = req.params.filename;
-    const contentType = mime.lookup(filename);
+    const contentType = mime.getType(filename);
     if (isFileStreamable(req, filesController)) {
       filesController.getFileStream(config, filename).then((stream) => {
         handleFileStream(stream, req, res, contentType);