// Reads a file into memory stream.  Common for all trials below
memoryStream = ReadDataIntoMemoryStream();

// TRIAL 1
FileStream fs= new FileStream();
memoryStream.CopyTo(fs);

string mimeType = "application/vnd.openxmlformats-
                   officedocument.spreadsheetml.sheet";

return File(fs, mimeType);

// TRIAL 2
string mimeType = "application/vnd.openxmlformats-
                   officedocument.spreadsheetml.sheet";
byte[] fileBytes = memoryStream.ToArray();
return File(fileBytes, mimeType);

// TRIAL 3
string filepath = ".......filename.xlsx";  // actual physical file on
                                              // the server
return Content(filepath);

$(document).ready(function () {
    $('#777').on('click', function () {
        $.ajax({
            method: "GET",
            url: "Report/createFile",
            contentType: "application/download",  // not needed for Trial 3
                                                  // where string is returned
            dataType:"text",
            success: function (data) {
            // window.location.href = data;  // THIS WORKS with TRIAL 3
                                             // where filepath is returned
             },
            error: function (data) {
                alert("Error.");
            },
        });
    });
});