Discover and include mime type when attaching files.

* Added mime type discovery to the `attach()` method so that the file mime type is sent on the request.
* Mime types are found using `finfo_open(FILEINFO_MIME_TYPE)` and then doing a `finfo_file()` on each of the attached files.
* Mime types will be used with the `curl_file_create()` function or with the regular `curl` `@`.
* Changed the test so that is uses a real file (an image), checking if the correct mime type is found and used.