Alternatively you can use jquery with a C# handler to upload a file. It will not cause any post back..

and

I can give you some links which will give you idea about avoiding page refresh please go through it:
Prevent Page Refresh in C#[^]
Detecting Page Refresh[^]
How to prevent page refresh in asp.net[^]
 
This question was already ask'd on this forum; see if you get some help from it:
how to avoid page refreshing while uploading file[^]