Images not loaded in email via API

 I have an image in signature. When sending from admin image is shown correctly and has attribute data-image-whitelisted. But when using API it is broken

$url = 'https://api2.frontapp.com/channels/' . $channel_id . '/messages';

		$body = array(
			'to'           => array( $to ),
			'subject'      => $subject,
			'body'         => $content,
			'text'         => wp_strip_all_tags( $content ),
			'options'      => array(
				'archive' => false,
			),
			'author_id'    => 'tea_j7twg',
			'signature_id' => $signature_id,
		);

		$args = array(
			'method'  => 'POST',
			'headers' => array(
				'Authorization' => 'Bearer ' . $api_token,
				'Content-Type'  => 'application/json',
				// 'Content-type'  => 'multipart/form-data',
			),
			'body'    => wp_json_encode( $body ),
			'timeout' => 30,
		);

I know also that if omit author_id, email always in spam.