How to save custom fields for attachments

I have added a custom field “Buy Now” link on media edit page but I am not able to save that data like other post types. Is there any other hook which is triggered after add/edit/update media/attachment?

I am using this function:

function update_attachment_extra_info( $post_id ) {

   // code to update data

add_action( 'save_post', 'update_attachment_extra_info' );

But this hook is not triggered.

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Here is an example that adds a custom media field named Buy Now. This example saves the value of the custom field on the media overlay screen via ajax, as well as the media edit screen (non ajax).

 * Add custom field to media
add_filter( 'attachment_fields_to_edit', 'wpse256463_attachment_fields', 10, 2 );
function wpse256463_attachment_fields( $fields, $post ) {
    $meta = get_post_meta( $post->ID, 'buy_now', true );
    $fields['buy_now'] = array(
            'label' =>  __( 'Buy Now', 'text-domain' ),
            'input' => 'text',
            'value' => $meta,
            'show_in_edit' => true,

    return $fields;         

 * Update custom field within media overlay (via ajax)
add_action( 'wp_ajax_save-attachment-compat', 'wpse256463_media_fields', 0, 1 );
function wpse256463_media_fields() {
    $post_id = $_POST['id'];
    $meta = $_POST['attachments'][ $post_id ]['buy_now'];
    update_post_meta( $post_id , 'buy_now', $meta );

    clean_post_cache( $post_id );

 * Update media custom field from edit media page (non ajax).
add_action( 'edit_attachment', 'wpse256463_update_attachment_meta', 1 );
function wpse256463_update_attachment_meta( $post_id ) {
    $buy_now = isset( $_POST['attachments'][ $post_id ]['buy_now'] ) ? $_POST['attachments'][ $post_id ]['buy_now'] : false;
    update_post_meta( $post_id, 'buy_now', $buy_now );