WordPress function to save custom meta box for attachment posts

sjf_deh_save_custom_meta.php

<?php

/** 
 * Save the Data
 *
 */  
function sjf_deh_save_custom_meta( $post_id ) {  

	// Check if the nonce exists.
	if( ! isset( $_POST[ 'sjf_deh_custom_meta_box_nonce' ] ) ) { return $post_id; }
	
	// Check if the nonce is valid.
	if( ! wp_verify_nonce( $_POST[ 'sjf_deh_custom_meta_box_nonce' ], basename( __FILE__ ) ) ) { return $post_id; }    

	// Dont' bother trying to do this if it's an autosave
	if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) { return $post_id; }

	// Check user permissions  
	if ( !current_user_can( 'edit_page', $post_id) ) { return $post_id; }  

	// Get the meta fields.
	$sjf_deh_meta_fields = sjf_deh_meta_fields();
      
    	// Loop through fields and save the data.
    	foreach ($sjf_deh_custom_meta_fields as $field) { 
    	
    		// Get the old value for this meta key.
		$old = get_post_meta($post_id, $field['id'], true);  
		
		// Get and sanitize the posted value for this meta key
		$new = sanitize_text_field( $_POST[ $field['id'] ] );  
		
		// If the new value is different, update the old value.
		if ( $new && $new != $old ) {
			update_post_meta($post_id, $field['id'], $new);  
		
		// If the new value is present and empty, delete the old value.
		} elseif ( '' == $new && $old ) {  
			delete_post_meta( $post_id, $field['id'], $old );  
		}
		
    	} // end foreach meta field  
}
add_action( 'edit_attachment', 'sjf_deh_save_custom_meta' );
add_action( 'add_attachment', 'sjf_deh_save_custom_meta' );