Follow up to 3bc535eb536e54451b722da9699d052c863ac1b4, determine the download mime by the extension rather than the actual download file (which will probably not exist). Refs #4344

refs #4592

Conflicts:
	classes/kohana/response.php
Merge branch '3.2/develop' of https://github.com/kohana/core into 3.2/develop