请尝试一下，以成功实现ajax

success: function () {
    window.location = '@Url.Action("DownloadAttachment", "PostDetail")';
}

更新的答案：

public ActionResult DownloadAttachment(int studentId)
{          
    // Find user by passed id
    // Student student = db.Students.FirstOrDefault(s => s.Id == studentId);

    var file = db.EmailAttachmentReceived.FirstOrDefault(x => x.LisaId == studentId);

    byte[] fileBytes = System.IO.File.ReadAllBytes(file.Filepath);

    return File(fileBytes, System.Net.Mime.MediaTypeNames.Application.Octet, file.Filename);

}

Ajax请求：

$(function () {
        $("#DownloadAttachment").click(function () {
            $.ajax(
            {
                url: '@Url.Action("DownloadAttachment", "PostDetail")',
                contentType: 'application/json; charset=utf-8',
                datatype: 'json',
                data: {
                    studentId: 123
                },
                type: "GET",
                success: function () {
                    window.location = '@Url.Action("DownloadAttachment", "PostDetail", new { studentId = 123 })';
                }
            });

        });
    });