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<?php
/**
 * Gravity Perks // GP Media Library // Auto-add Custom Field for File ID
 * http://gravitywiz.com/documentation/gp-media-library/
 */
add_filter( 'gform_post_data', function( $data, $form, $entry ) {

	if( ! is_callable( 'gp_media_library' ) ) {
		return $data;
	}

	foreach( $form['fields'] as &$field ) {
		if( $field->type == 'post_custom_field' && gp_media_library()->is_applicable_field( $field ) ) {
			$meta_key = $field->postCustomFieldName;
			$id_meta_key = "{$meta_key}_id";
			$file_id = gp_media_library()->get_file_ids( $entry['id'], $field->id, 0 );
			$data['post_custom_fields'][ $id_meta_key ] = $file_id;
		}
	}

	return $data;
}, 10, 3 );