I have an HTML form that allows users to fill out a bunch of different parameters, and then it will display the result on the same page via an AJAX call. Pretty standard, and it works great. I had a request to allow for exporting the data in Excel format. Unfortunately, just returning a File ActionResult won't work from within the context of an Ajax form. There were some workaround posted out there (e.g. hidden iframe, duplicate the form on the page, etc.). I found the following approach to be a nice compromise and DRYs things up considerably. The key is that by putting the onclick on the Export button to do this.form.submit(), it forces the request to be a standard POST and not an AJAX call.

View.cshtml

@using(Ajax.BeginForm("ReportData", new AjaxOptions {UpdateTargetId="result"})) {
  ... other input elements here
  <input type="submit" value="Display" />
  <input type="submit" value="Export" onclick="this.form.submit();">
}

Controller.cs

public ActionResult ReportData(... parameters go here ...) {
  if (Request.IsAjaxRequest()) {
    // Do the normal behavior here to build up a string
    return Content(s);
  }

  // I actually stream the result with a custom ExcelResult action, 
  // but this shows how to just return a file.
  return File(@"c:\library\report.xls", "application/vnd.ms-excel");
}