MIME type property null in some cases



I am writing a form attachment object in designer using the normal javascript method:

var formObj = event.target;


Which is fine. But I want to allow only word or pdf files, so I use

fileObj.MIMEType == "application/msword";

to see if it is a word document for instance. The problem is that MIMType  is returning null when I run it with Acrobat Pro 9. When running the form under Reader 9 & Reader 10 it returns the mime correctly.

The form is reader extended through LiveCycle ES and each time I rendered the same form SmartForm Centre, so Im certain it's the same code. Anyone ever come across this before?