can .xhtml be validated?

Dear validator,
       can file with .xhtml extension be validated?  some weeks ago, I
validated my .xhtml file as attached through validator.w3.org, but today I
try the same file, it said

"Sorry! This document can not be checked."

and after I rename the file to .html, it passed the validation.  Can you
please let me know what's the problem?
I did want to change the file extension  since it will cause link problem.


many thanks in advanced.

SH,
from  China
hangzhouwandaqiao@gmail.com