Transferring files with p5 js, node js, socket io

Hello! I am using p5 js for developing a very simple chat platform. I am also using node and socket.
The video series I am following created by Coding Train

I implemented the chatting concept successfully. But now I wanted to implement the uploading and downloading system. The idea is that a user will drag and drop a file and I will send it to the server (node js) and then broadcast it to all the other users who will download it then.

I watched this video - Drag and Drop by Coding Train

But as I am a complete beginner… I could not solve it.
So how do I do it? Or even if I am able to broadcast the file, how do I save the file once recieved?

Thanks in advance! I will really be grateful if you answer it.