WordPress: Allow the user to only see media they've uploaded.

wp-restrict-user-media.php

add_action('pre_get_posts','users_own_attachments');
function users_own_attachments( $wp_query_obj ) {

    global $current_user, $pagenow;

    if( !is_a( $current_user, 'WP_User') )
        return;

    if( !in_array( $pagenow, array( 'upload.php', 'admin-ajax.php' ) ) )
        return;

    if( ! current_user_can ( 'administrator' ) && ! is_admin () )
        $wp_query_obj->set('author', $current_user->ID );

    return;
}

add_filter( 'ajax_query_attachments_args', 'show_current_user_attachments', 10, 1 );

function show_current_user_attachments( $query = array() ) {
    $user_id = get_current_user_id();
    if( $user_id ) {
        $query['author'] = $user_id;
    }
    return $query;
}