the updatepanel does not support <input type="file"> except under
certain conditions. if a file is selected, you must do a postback, not
an ajax call. this means the submit button is not a trigger for the
panel. if the trigger is selected, then the filename will be lost and
must be reselected when the panel repaints.

this is because ajax cannot upload a file, only a browser submit can.

-- bruce (sqlwork.com)