The file metadata microservice project allows the user to upload a file and the application will return to the user the name, size, and location of that file.
This application was one of the easiest of all of the backend API challenges. At this point, setting up the Node/Express app was simple. The only catch was learning how to use the multer package that provides methods for measuring metadata about files. Aside from that, the actual functionality of this application was 9 lines long!
The Server-side Development with NodeJS course on Coursera helped me complete this project. It covers Node, Express, MongoDB, and Mongoose, so you can use it to learn everything you might need for the M, E, and N parts of the MEAN stack.