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I have a valid XHTML page (checked by "Validate by URL" service). If I use the "Validate by File Upload" service on the same XHTML page I get the following error: "Sorry, I am unable to validate this document because its content type is text/plain, which is not currently supported by this service." I have tried this with several valid XHTML pages - all give the same error. Don't know if this is relevant but I've recently installed Windows XP Service Pack 2 - I've only noticed this error since applying the SP.
*** This bug has been marked as a duplicate of 838 ***