In Node.js, given a URL, how do I check whether its a jpg/png/gif?

Question

My current method is this:

var request = require('request');
var mime = require('mime');
var fs = require('fs');
var uri = 'http://www.sweetslyrics.com/images/img_gal/25646_christina-perri-213968.jpg';
request({
        'method':'GET',
        'uri': uri
},function(err, response,body){
    var tmp_path = '/tmp/123456';
    fs.writeFile(tmp_path, body, function(err) {
        console.log(mime.lookup(tmp_path));  //application/octet-stream ?????
    });
});

The image is obviously a picture, but node-mime says it's application/octet-stream. Why?

Note: - I do not want to rely on the Response Headers content-type, because based on my experience, sometimes those response headers are set incorrectly...and they do not determine the true file type. (that's why I save it to a file, and then have node-mime determine it for me!)

I want to know the best way to determine if a file is an image, with 0 margin of error.

Edit: I just realized that node-mime isn't "magic". It just checks for the extension :( ...

Edit2: I found this: https://github.com/SaltwaterC/mime-magic

Answer 1

Just read the first bytes of the stream, and check it for the so called "magic number".

Magic numbers are the first bits of a file which uniquely identify the type of file.

For example:
-Every JPEG file begins with ff d8 (hex).
-Every png file begins with a 89 50 4e 47.
-There is a comprehensive table of magic numbers here

This way even if you have a file without extension you can still detect its type.
Hope this helps.