More permissive regex for mimeType

The regex used for mimeType is too restrictive. This change allows for values that meet the criteria described in [RFC 2045](http://tools.ietf.org/html/rfc2045) and [RFC 2046](http://tools.ietf.org/html/rfc2046).

Hat tip to @mnot for his regex, which he [posted here](http://lists.w3.org/Archives/Public/xml-dist-app/2003Jul/0064.html).