.net Website throws 404 error

Question

I've got a working website that I can upload and download files to a folder within the project locally. However when I use IIS to host the website across LAN, I get a 404 error when trying to download files - I can upload the files fine and they appear in the correct folder.

Error:

HTTP Error 404.0 - Not Found The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.

Here are my controllers.

Download controller:

[HttpGet]
public virtual ActionResult Download(string file)
{
    string fullPath = Path.Combine(Server.MapPath("~/SupportAttachments/"), file);
    return File(fullPath, "application/vnd.openxmlformats-officedocument.wordprocessingml.document", file);
}

Upload Controller:

[HttpPost]
public JsonResult UploadFiles(string id)
{
    try
    {
        foreach (string file in Request.Files)
        {
            var hpf = Request.Files[file] as HttpPostedFileBase;
            if (hpf.ContentLength == 0)
                continue;
            var fileContent = Request.Files[file];
            if (fileContent != null && fileContent.ContentLength > 0)
            {
                // get a stream
                var stream = fileContent.InputStream;
                // and optionally write the file to disk
                var fileName = id + " " + hpf.FileName;
                var path = Path.Combine(Server.MapPath("~/SupportAttachments/"), Path.GetFileName(fileName));

                // Save the file
                hpf.SaveAs(path); 
            }
        }
    }
    catch (Exception)
    {
        Response.StatusCode = (int)HttpStatusCode.BadRequest;
        return this.Json("Upload failed");
    }

    return this.Json("File uploaded successfully");
}

Can anyone advise me as to why I can't download files when hosting via IIS?