How to show 'in progress' while a FileResult function is running in MVC4

I have a download button that calls this function:

public FileResult DownloadExport()
{
    string fileName = "example";

    // Activate 'In progress'
    // Call to a function that takes a while
    // Deactivate 'In progress'

    return File(fileName, System.Net.Mime.MediaTypeNames.Application.Octet, Path.GetFileName(fileName));
}

So, I call a function that generates a file for me. This function takes a while and I don't want my users to think the application crashed. That's why I want to show 'In progress' while the user is waiting. How can I implement this?

For clarification: This question is not about the progress of the download, but about the progress of the function generating the file.