Base64 Encode - For CF7

// paste this code inside your wp-content/themes/{themename}/functions.php
add_action('wpcf7_posted_data', function( $posted_data ){

    // Subbmisson instance from CF7
    $submission = WPCF7_Submission::get_instance();

    // Make sure we have the data
    if ( ! $posted_data ) {
        $posted_data = $submission->get_posted_data();
    }

    // @note : change the name of your upload field.
    $file_upload_name = 'uploader-249';

    if( isset( $posted_data[ $file_upload_name ] ) ) {
        foreach( $posted_data[ $file_upload_name ] as $index => $file ) {
            if( wpcf7_is_url( $file ) ) {
                $posted_data["$file_upload_name"][$index] = base64_encode( file_get_contents( $file ) );
            }
        }
    }

    return $posted_data;

},100, 1 );