I was sending a file through a form, and after using a form with multipart/form-data, server-side I had the File objects in req.files.

This gave me some information, like the path, name, size, type and so on:

{
  logo: File {
    size: 121920,
    path: '/var/folders/tn/h8lfq1sj7c33c0p30qgkd3mw0000gn/T/upload_b9e85b7cf989482a1760d82b77fd555a',
    name: 'Screen Shot 2021-06-07 at 21.40.29.png',
    type: 'image/png',
    hash: null,
    lastModifiedDate: 2021-06-07T22:20:50.150Z,
//...
  }
}

Notice the temp file path does not have an extension.

If you user the name server-side, no problem. But I wanted to change it and use my own naming conventions, so I needed just the file extension.

To get it, you can do 2 things: parse the file name, with

const path = require('path')
path.extname(req.files.logo.name) //.png

which does not require any 3rd party library.

Or you can use the mime-types package and look at the MIME type:

const mime = require('mime-types')
mime.extension('text/plain') //txt
mime.extension('image/png') //png