How to upload a file to WordPress

wordpress-upload.php

<?php 
/**
 * Handle file uploading and add attachment.
 *
 * @return array Uploaded file's details on success, error message on failure
 */
function wp_import_handle_upload() {
	if ( !isset($_FILES['filefield_name']) ) {
		$file['error'] = __( 'File is empty. Please upload something more substantial. This error could also be caused by uploads being disabled in your php.ini or by post_max_size being defined as smaller than upload_max_filesize in php.ini.' );
		return $file;
	}

	$overrides = array( 'test_form' => false, 'test_type' => true );
	$file = wp_handle_upload( $_FILES['filefield_name'], $overrides );

	if ( isset( $file['error'] ) )
		return $file;

	$url = $file['url'];
	$type = $file['type'];
	$file = $file['file'];
	$filename = basename( $file );

	// Construct the object array
	$object = array(
		'post_title' => $filename,
		'post_content' => $url,
		'post_mime_type' => $type,
		'guid' => $url,
		'context' => 'import',
		'post_status' => 'private'
	);

	// Save the data
	$id = wp_insert_attachment( $object, $file );

	// schedule a cleanup for one day from now in case of failed import or missing wp_import_cleanup() call
	wp_schedule_single_event( time() + 86400, 'importer_scheduled_cleanup', array( $id ) );

	return array( 'file' => $file, 'id' => $id );
}