Fixed an issue in MIME type parsing that would cause it to fail whenever...

Calling `asInstanceOf` (without a type parameter) on an instance of `String` will always fail, since instances of `String` cannot be cast to instances of `Nothing`.

This caused MIME type parsing to fail whenever a MIME type contained a parameter (such as `charset=utf-8`, for example).

Removing the offending call seems to fix the issue.