can't download a .mobi file from server even though it exists (works for pdf and txt)

 i have a site that will enable users to download an ebook file
i have successfully built architecture to enable users to download pdf and txt files, but it won't work when i try with a .mobi file, even though i know the file exists.
the site works by allowig users to upload their work as .doc, .docx or .odt. once it's verified, the site publishes it as .pdf, .txt and .mobi and makes it available for download. the download code is...
string mstitle = title.ToUpper();
        string dlpath = [the server path] + msid + mstitle + ".mobi";
        string attachment = "attachment; filename=" + msid + mstitle + ".mobi";        HttpWebRequest objRequest = (HttpWebRequest)WebRequest.Create(dlpath);        HttpWebResponse objResponse = (HttpWebResponse)objRequest.GetResponse();  &nb ...