How to display .gif and .svg files to the client using Node.js?

I am trying to figure out a way using just Node.js to display image on the client's browser. With my code I can only make .png files display , .svg and .gif files get downloaded instead of displayed. Why is this?

I am using this:

  var http = require('http');
  var fs = require('fs');
  var path = require('path');
  var server = http.createServer(function(req,res) {

if(path.extname(req.url) == ".gif") {
    fs.readFile(path.basename(req.url), function(err, data) {
    res.writeHead(200, {'content-type':'image/gif'});
    res.end(data);

    });
}


});
server.listen(80);

I tested .gif .png. .svg , also changing the content type to the correct file extension name. PNG displays and the rest just get downloaded.

Not looking for a framework solution like Express I want to understand how node.js works and why this is happening.