wp_insert_attachment.php

function krava_insert_attachment( $file_handler, $post_id, $setthumb = 'false' ){
			// check to make sure its a successful upload
			if ($_FILES[$file_handler]['error'] !== UPLOAD_ERR_OK) __return_false();
			
			require_once(ABSPATH . "wp-admin" . '/includes/image.php');
			require_once(ABSPATH . "wp-admin" . '/includes/file.php');
			require_once(ABSPATH . "wp-admin" . '/includes/media.php');
			$attach_id = media_handle_upload( $file_handler, $post_id );
			if ($setthumb) update_post_meta( $post_id, '_thumbnail_id', $attach_id );
			
			return $attach_id;
		}
		if ( $_FILES ) {
			$files = $_FILES['files'];
			foreach ( $files['name'] as $key => $value ) {
				if ( $files['name'][$key] ) {
					$file = array(
						'name'     => $files['name'][$key],
						'type'     => $files['type'][$key],
						'tmp_name' => $files['tmp_name'][$key],
						'error'    => $files['error'][$key],
						'size'     => $files['size'][$key]
					);
					$_FILES = array("files" => $file);
					foreach ( $_FILES as $file => $array ) {
						$newupload = krava_insert_attachment( $file, $post_id );
					}
				}
			}
		}