FileUpload.PostedFile在UpdatePanel中始终为null

I have a FileUpload control inside an UpdatePanel. I have successfully registered the upload button with the scriptmanager to do a full postback. The only issue I have is that FileUpload.HasFile is always returning null. Note: When I move the FileUpload control outside of the updatepanel everything works file. Is there a solution or workaround for this.I have a FileUpload control inside an UpdatePan