How to change the state of a button after file download using Response.writefile

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ASP.NET

Hi all,
I am downloading a file from web server using "Response". What I want is I have to enable a button after successful download which is not happening. Help me in doing this.

Below is the code.

string filepath = Serverpath + validfile;
System.IO.FileInfo file = new System.IO.FileInfo(filepath);
Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name);
Response.AddHeader("Content-Length", file.Length.ToString());
Response.ContentType = "application/octet-stream";
Response.WriteFile(file.FullName);
btnCreate.Enabled=true;/// this is not happening.

Thanks in advance.