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,
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