Request: file uploads #133

Part of #103. Line numbers refer to branch chore/php-8.4-dependabot (#98). Verified = reproduced, suspected = from reading the code.

• file() overwrites the content type, even if the body is JSON or a content type was set explicitly (RequestBuilder.php:246). Verified.
• Multiple uploads under one field keep only the last file (docs[]) (RequestBuilder.php:258). Verified.
• The client MIME type is always application/octet-stream and can't be set (RequestBuilder.php:263-283). Verified.
• Temp files under sys_get_temp_dir() are never deleted. Clean them up in #[After].