I have this ajax:

 $('.getid').on('click', function(){
        var imagename =  $(this).data("id");
        // you can make ajax call here to get data
        $.ajax({
        type: "POST",
        url: "Monitor/downloadDesiredImage",
        data: {'val' : imagename},
        dataType: "text"
        }).done(function(data) {
       console.log(data);        
       }).fail(function() {
    alert( "error" );
  });
});

This will work with a button that will send on.click a id value saved in a var imagename, that is a string that correspond to a image name saved on my server.

This is the php function that will get that id :

public function downloadDesiredImage() {
        $imagename = $_POST['val'];
        echo "this is my ".  $imagename;
        $file = file_get_contents('./images/'.$imagename.'.jpg', FILE_USE_INCLUDE_PATH);
        $imageData = base64_encode(($file));
        $src = 'data: '.mime_content_type($file).';base64,'.$imageData;
        echo '<img src="' . $src . '">';

    }

That will output the image in the network response, but I will also get the error :

Message: mime_content_type(): Invalid path

But what I want to do is after I click the button is to download that image from my server. How I can do that ? Thanks!