Validator.nu Form Upload Input
From WHATWG Wiki
To use Validator.nu as a Web service by form-based file upload, the client issues an HTTP request either to http://validator.nu/ or http://html5.validator.nu/ using the POST method with an multipart/form-data entity body. Common parameters are encoded as form fields. The document to check is included as the value of a file upload field called file. The file field must be the last field in the submission.
Since the Content-Type supplied by browsers for the uploaded file is unreliable, the client-supplied Content-Type is overridden with a synthetic Content-Type if the filename is supplied and it has a well-known extension. The well-known extensions are html, htm, xhtml, xht, atom, rng, xsl, xml and dbk.
The parameter field values must decode as UTF-8.
