Extending the WordPress Media Uploader: Custom Fields | http://www.ibenic.com/extending-wordpress-media-uploader-custom-fields

att.html

<input type="text" name="attachments[123][field_name]" />

custom.php

<?php

/**
 * Adding a custom field to Attachment Edit Fields
 * @param  array $form_fields 
 * @param  WP_POST $post attachment post object       
 * @return array              
 */
function ibenic_add_media_custom_field( $form_fields, $post ) {
    // Adding the form fields
    return $form_fields;
}
add_filter( 'attachment_fields_to_edit', 'ibenic_add_media_custom_field', null, 2 );

info.php

<?php

/**
 * Adding a custom field to Attachment Edit Fields
 * @param  array $form_fields 
 * @param  WP_POST $post        
 * @return array              
 */
function ibenic_add_media_custom_field( $form_fields, $post ) {
    $field_value = get_post_meta( $post->ID, 'info', true );

    $form_fields['info'] = array(
        'value' => $field_value ? $field_value : '',
        'label' => __( 'Info' ),
        'helps' => __( 'Enter some additional info' ),
        'input'  => 'textarea'
    );

    return $form_fields;
}
add_filter( 'attachment_fields_to_edit', 'ibenic_add_media_custom_field', null, 2 );

info_save.php

<?php

/**
 * Saving the attachment data
 * @param  integer $attachment_id 
 * @return void                
 */
function ibenic_save_attachment( $attachment_id ) {
    if ( isset( $_REQUEST['attachments'][ $attachment_id ]['info'] ) ) {
        
        $info = $_REQUEST['attachments'][ $attachment_id ]['info'];
        update_post_meta( $attachment_id, 'info', $info );
    
    }
}
add_action( 'edit_attachment', 'ibenic_save_attachment' );

save.php

<?php

/**
 * Saving the attachment data
 * @param  integer $attachment_id 
 * @return void                
 */
function ibenic_save_attachment( $attachment_id ) {
    // Saving the data
}
add_action( 'edit_attachment', 'ibenic_save_attachment' );

/**
 * @param array $post
 * @param array $attachment
 * @return array
 */
function ibenic_fields_to_save( $post, $attachment ) {
    // Check the field in $attachment and save if needed
    return $post;
}
add_filter( 'attachment_fields_to_save', 'ibenic_fields_to_save', 20, 2 );

tags.php

<?php

$_POST['attachments'][att_id]['tags] = array(
  1 => 'adminstrator',
  2 => 'another_user',
  ...
);

users.php

<?php

/**
 * Adding a custom field to Attachment Edit Fields
 * @param  array $form_fields 
 * @param  WP_POST $post        
 * @return array              
 */
function ibenic_add_media_custom_field( $form_fields, $post ) {
    $field_value = get_post_meta( $post->ID, 'info', true );
  
    $form_fields['info'] = array(
        'value' => $field_value ? $field_value : '',
        'label' => __( 'Info' ),
        'helps' => __( 'Enter some additional info' ),
        'input'  => 'textarea'
    );
    
    // Getting the tags
    $tags = get_post_meta( $post->ID, 'tags', true );
    // Getting all the users
    $users = get_users();
    $tag_html = '';
    foreach ( $users as $user ) {
        // Adding the checkbox HTML with the 'checked="checked"' attribute if the user ID is a tag key
        $tag_html .= '<input ' . checked( array_key_exists( $user->ID, $tags ), true, false ) . ' type="checkbox" name="attachments[' . $post->ID . '][tag][' . $user->ID . ']"> ' . $user->display_name . '<br/>';
    }
    
    // Adding the tag field
    $form_fields['tag'] = array( 
        'label' => __( 'Tags:' ),
        'input'  => 'html',
        'html' => $tag_html
    );
  
    return $form_fields;
}
add_filter( 'attachment_fields_to_edit', 'ibenic_add_media_custom_field', null, 2 );

users_save.php

<?php

/**
 * Saving the attachment data
 * @param  integer $attachment_id 
 * @return void                
 */
function ibenic_save_attachment( $attachment_id ) {
    if ( isset( $_REQUEST['attachments'][ $attachment_id ]['info'] ) ) {
        
        $info = $_REQUEST['attachments'][ $attachment_id ]['info'];
        update_post_meta( $attachment_id, 'info', $info );
    
    }

    if ( isset( $_REQUEST['attachments'][ $attachment_id ]['tag'] ) ) {
        
        $tags = $_REQUEST['attachments'][ $attachment_id ]['tag'];
        update_post_meta( $attachment_id, 'tags', $tags );
    
    }
}
add_action( 'edit_attachment', 'ibenic_save_attachment' );