"content type is text/plain"

After installing Service Pack 2, this is the message I am receiving when
trying to validate a html file that worked correctly before the upgrade.
What can be done to rectify this?  The file has not been changed, just
SP2 installed.
 
Error message:
 
"Sorry, I am unable to validate this document because its content type
is text/plain, which is not currently supported by this service. 
 
The Content-Type field is sent by your web server (or web browser if you
use the file upload interface) and depends on its configuration.
Commonly, web servers will have a mapping of filename extensions (such
as ".html") to MIME Content-Type values (such as text/html). 
 
That you recieved this message can mean that your server is not
configured correctly, that your file does not have the correct filename
extension, or that you are attempting to validate a file type that we do
not support yet. In the latter case you should let us know that you need
us to support that content type (please include all relevant details,
including the URL to the standards document defining the content type)
using the instructions on the Feedback Page."
 
Thanks for your help.
 
Shaun Beaty