file-upload-handler.php

<?php /* This function attaches the image to the post in the database, add it to functions.php */

function 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;
} ?>

process-post.php

<?php /* Also in the template file, this script processes any $_POST data and hands off the images to the insert_attachment() function. Again you may want to wrap this in a current_user_can conditional, and probably do some sanity checks on incoming $_POST data. */

global $post;

if ( $_FILES ) {
	$files = $_FILES['upload_attachment'];
	foreach ($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("upload_attachment" => $file);

			foreach ($_FILES as $file => $array) {
				$newupload = insert_attachment($file,$post->ID);
			}
		}
	}
} ?>

upload-form.html

<!-- This is the form that you add to the template file. You probably want to wrap this in a current_user_can conditional. -->

<form action="#" method="post" enctype="multipart/form-data"><input type="file" name="upload_attachment[]" /> <input type="submit" value="Upload" /></form>