Is contact form 7 unable to access my uploads folder?

Like I mentioned in a previous question, I want to send an email attachment dynamically when a user gives me there contact details.

I started with the absolute url:

get_template_directory().'/test.pdf'

This works fine, but when I try to reference a file in my uploads folder, it does not work.

Is contact form 7 unable to access my uploads folder?

Okay, so here's my code:

add_action( 'wpcf7_before_send_mail', 'send_pdf' );
function send_pdf( $cf7 ) {
    $id = $cf7->id();
    if ($cf7->id==1308){
        $submission = WPCF7_Submission::get_instance();
        $submission->add_uploaded_file('pdf', 'http://ift.tt/1D3JMYG');
    }
}

The url here does not want to work, I get an email but no pdf, with the other url though it works fine.

What gives?