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
).