fileUpload contron inside ajax updatePanel

[elic05]

I have on a form

ajax updatePanel

and inside the ajax updatePanel
a few fields
and fileUpload
and a button


I press the button after i browsed my pc to locate a file

and because the content of the textBox of the fileUpload becomes empty
the statement

if FileUpload1.HasFile

returns false

why?
and what can I do to use ajax and to upload file?