Strange Problem with regular expression and a button with property causesvalidation=false

I have a fileupload control over which i have two validators one is required field validator and other one is a regular expression validator to check for specific file uploads.

I have two buttons One is for submitting the file and other one is for Clearing all fields in the form. Button for clearing the forms have a property CausesValidation="False" so that regular expression does not come in between the process of clearing the fields.

Now Problem i am facing is that when in Fileupload control user inputs a file type which is not allowed and  then if "Clear All button is pressed", All fields are cleared with no hassles. Everthing runs fine uptil here.

BUT

If user goes onto write anything in the fileupload control which is not as per the correct format of regular expression validator for eg(user inputs "d:fsdf") , message is shown correctly ...