If Media Type .GIF

I’m not quite sure you’re using the wp_check_filetype() correctly. I just successfully tried the following to determine if the post thumbnail ext was jpg or not:

$url = wp_get_attachment_url( get_post_thumbnail_id( ) );
$filetype = wp_check_filetype($url);
    if ($filetype[ext] == 'jpg') {
        echo 'this is gif';
    }
    else {
       echo 'this aint gif';
    }

You probably know already but if you ever want to see what the function returns, you can use echo print_r($filetype) and it will print the returned array and you can dissect how to best use the returned information (sometimes easier than interpreting up the codex!)