What is the function got get all the media files wordpress?

Uploaded images are stored as posts with the type "attachment"; use get_posts() with the right parameters. In the Codex entry for get_posts(), this example:

<?php$args = array(    'post_type' => 'attachment',    'numberposts' => -1,    'post_status' => null,    'post_parent' => null, // any parent    ); $attachments = get_posts($args);if ($attachments) {    foreach ($attachments as $post) {        setup_postdata($post);        the_title();        the_attachment_link($post->ID, false);        the_excerpt();    }}?>

...loops through all the attachments and displays them.

If you just want to get images, as TheDeadMedic commented, you can filter with 'post_mime_type' => 'image' in the arguments.

<ul>            <?php if ( have_posts() ) : while ( have_posts() ) : the_post();                        $args = array(                        'post_type' => 'attachment',                        'numberposts' => -1,                        'post_status' => null,                        'post_parent' => $post->ID                        );                    $attachments = get_posts( $args );                    if ( $attachments ) {                        foreach ( $attachments as $attachment ) {                            echo '<li>';                            echo wp_get_attachment_image( $attachment->ID, 'full' );                            echo '<p>';                            echo apply_filters( 'the_title', $attachment->post_title );                            echo '</p></li>';                        }                    }            endwhile; endif; ?>        </ul>