I tried to search for the problem in the internet and I see everyone is asking about the problem for FileUpload control inside the UpdatePanel. First of all, I am not using an U
I found the answer. The txt file I was uploading was empty. I edited the text file and then saved by typing something in it. I could not find this anywhere mentioned in the msdn or I am not sure whether I was looking at a wrong place. The suggestion by Kasys in this post helped me.