Validating files with regularexpressionvalidator is always false

I would like to use a regularexpressionvalidator to ensure that a file that a
user is uploading is a certain file type. I tried the following example,

http://msdn2.microsoft.com/en-us/library/aa479405.aspx

However, If you browse to a file that is not allowed, and then browse to
another file that is allowed and click on a submit button, page.isvalid is
still false even though a valid file has been selected. Any help would be
greatly appreciated.