Update Panel And Response.End

Hi, I'm relatively new to asp.net and I've encountered a 
problem, I have a button that clicks on a file and generates another output, 
the problem is that when I click I want to show a simple gif of loading something 
that I have already achieved but the problem is that the response.
end that the user sends the file to download is not done, I have investigated and so far
 I only do one thing or another, someone has an idea of ​​how to do that?