I think... after browse your file postback event called in your page that why your fileupload control lost it file name ...
So you have to check that..
and always check Hasfile condition before uploading file on server..
For Example :
if (FileUpload.HasFile)
{
string filename=System.IO.Path.GetFileName(FileUpload.PostedFile.FileName);
FileUpload.SaveAs(Server.MapPath("img/" + filename));
}
Thanks
AARIF SHAIKH