WordPress: Upload image to REST API via multipart/form-data body

wp-multipart-image-upload.php

<?php

/**
 * Creates a multipart/form-data body for an image and form fields.
 * 
 * @param string $file_path The path to the file.
 * @param string $filename  The base filename, if different from the file path.
 * @param array  $fields    The form fields.
 * @return array The boundary ID and the body.
 */
function create_multipart_form_data( string $file_path, string $filename = '', array $fields = [] ): array {

	$boundary = wp_generate_password( 24, false, false );

	$body = '';

	foreach ( $fields as $name => $value ) {
		$body .= '--' . $boundary . "\r\n";

		// Such as the "meta" field for post meta.
		if ( is_array( $value ) ) {
			foreach ( $value as $sub_name => $sub_value ) {
				$body .= '--' . $boundary . "\r\n";
				$body .= 'Content-Disposition: form-data; name="' . $name . '[' . $sub_name . ']"' . "\r\n\r\n";
				$body .= $sub_value . "\r\n";
			}
		} else {
			$body .= '--' . $boundary . "\r\n";
			$body .= 'Content-Disposition: form-data; name="' . $name . '"' . "\r\n\r\n";
			$body .= $value . "\r\n";
		}
	}

	if ( empty( $filename ) ) {
		$filename = basename( $file_path );
	}

	$body .= '--' . $boundary . "\r\n";
	$body .= 'Content-Disposition: form-data; name="file"; filename="' . $filename . '"' . "\r\n";
	$body .= 'Content-Type: ' . mime_content_type( $file_path ) . "\r\n\r\n";
	$body .= file_get_contents( $file_path ) . "\r\n";
	$body .= '--' . $boundary . '--';

	return [
		'boundary' => $boundary,
		'body'     => $body,
	];
}

// Example code.
// Post meta can be sent in the "meta" field, but it needs to be registered via
// register_post_meta(), see example.
$form_fields = [
	'post_title' => 'Image title',
	'post_name'  => 'custom-slug',
	'meta'       => [
		'test_hello' => 'world',
	],
];

$multipart = create_multipart_form_data( $image_filename, $filename, $form_fields );

$headers['Content-Type'] = 'multipart/form-data; boundary=' . $multipart['boundary'];

$args = [
	'headers'   => $headers,
	'body'      => $multipart['body'],
	'sslverify' => false,
	'timeout'   => 30,
];

$response = wp_remote_post( $image_upload_endpoint, $args );

// Example of register_post_meta().
// register_post_meta(
// 	'attachment',
// 	'test_hello', [
// 		'type'         => 'string',
// 		'description'  => 'Test meta field.',
// 		'single'       => true,
// 		'show_in_rest'      => [
// 			'schema' => [
// 				'type' => 'string',
// 			],
// 		],
// 	]
// );