Document can not be checked

Dear Readers,
I am writing to you because I was not able to find the answer after
searching the FAQ and list archives.
I am using your file upload interface to check a page called
default.aspx because our server is behind a firewall.
The resulting error says:
Top of Form 1
Result:	Failed validation 		
File:	K:\SD_Intranet_V1\default.aspx		
Encoding:			
Doctype:			
Bottom of Form 1
Sorry! This document can not be checked.

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 received 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.

Could you help by providing instructions for changing the Content-Type
field sent by the web browser in question (Internet Explorer 6)? I am
using Windows XP SP2 so I followed the suggestions on the information
page (http://www.w3.org/QA/2005/01/Validator-IE_WinXP_SP2) assuming the
problem is only with IE6. When I tried in Mozilla Firefox 1.5.08 I got
the same erorr but with a different content type
(application/octet-stream):

Result:	 Failed validation 	
File:	default.aspx	 
Encoding:		
Doctype:		

Sorry! This document can not be checked.

Sorry, I am unable to validate this document because its content type is
application/octet-stream, 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 received 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.


Is there a downloadable version or do you recommend any of the
downloadable validators that could be run locally?

Regards,

Keith Kubicek
Information Systems Specialist

NOTE: Whenever possible, give the address of the document you were
checking.