media_handle_sideload() │ WP 2.6.0

Handles a side-loaded file in the same way as an uploaded file is handled by media_handle_upload().

Uses: wp_handle_sideload(), wp_insert_attachment(), wp_read_image_metadata(), wp_update_attachment_metadata(), wp_generate_attachment_metadata()
Used By: media_sideload_image()

No Hooks.

Return

Int|WP_Error. The ID of the attachment or a WP_Error on failure.

Usage

media_handle_sideload( $file_array, $post_id, $desc, $post_data );
$file_array(string[]) (required)
Array that represents a $_FILES upload array.
$post_id(int)
The post ID the media is associated with.
$desc(string)
Description of the side-loaded file.
Default: null
$post_data(array)
Post data to override.
Default: empty array

Changelog

Since 2.6.0 Introduced.
Since 5.3.0 The $post_id parameter was made optional.

Code of media_handle_sideload() WP 5.9.3

wp-admin/includes/media.php 
function media_handle_sideload( $file_array, $post_id = 0, $desc = null, $post_data = array() ) {
	$overrides = array( 'test_form' => false );

	if ( isset( $post_data['post_date'] ) && substr( $post_data['post_date'], 0, 4 ) > 0 ) {
		$time = $post_data['post_date'];
	} else {
		$post = get_post( $post_id );
		if ( $post && substr( $post->post_date, 0, 4 ) > 0 ) {
			$time = $post->post_date;
		} else {
			$time = current_time( 'mysql' );
		}
	}

	$file = wp_handle_sideload( $file_array, $overrides, $time );

	if ( isset( $file['error'] ) ) {
		return new WP_Error( 'upload_error', $file['error'] );
	}

	$url     = $file['url'];
	$type    = $file['type'];
	$file    = $file['file'];
	$title   = preg_replace( '/\.[^.]+$/', '', wp_basename( $file ) );
	$content = '';

	// Use image exif/iptc data for title and caption defaults if possible.
	$image_meta = wp_read_image_metadata( $file );

	if ( $image_meta ) {
		if ( trim( $image_meta['title'] ) && ! is_numeric( sanitize_title( $image_meta['title'] ) ) ) {
			$title = $image_meta['title'];
		}

		if ( trim( $image_meta['caption'] ) ) {
			$content = $image_meta['caption'];
		}
	}

	if ( isset( $desc ) ) {
		$title = $desc;
	}

	// Construct the attachment array.
	$attachment = array_merge(
		array(
			'post_mime_type' => $type,
			'guid'           => $url,
			'post_parent'    => $post_id,
			'post_title'     => $title,
			'post_content'   => $content,
		),
		$post_data
	);

	// This should never be set as it would then overwrite an existing attachment.
	unset( $attachment['ID'] );

	// Save the attachment metadata.
	$attachment_id = wp_insert_attachment( $attachment, $file, $post_id, true );

	if ( ! is_wp_error( $attachment_id ) ) {
		wp_update_attachment_metadata( $attachment_id, wp_generate_attachment_metadata( $attachment_id, $file ) );
	}

	return $attachment_id;
}

upload download (file system)