Force download not working for MS Office files

I have created a JSP page to force the download of different types of files.  PDF, TXT and ZIP files download perfectly, however DOC, XLS PPT and TIFF files do not - they download but then are not correctly interpreted by the application.  A Word doc, for example, looks like:

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For the XLS, DOC and PPT files, I get the same result if I set the MIME Type to APPLICATION/OCTET-STREAM, APPLICATION/STREAM, and APPLICATION/UNKNOWN.  I've confirmed that there is nothing wrong with the files themselves.  Interestingly, if I zip the file it downloads AND opens correctly!

Here's my code (The code comes largely from a number of answers on this site - thanks especially to cheekyci):

<%@ page import="java.util.*,java.io.*"%>
<%
  String filePath = this.getServletContext().getRealPath("/") + "/doc/";
  String fileName = request.getParameter("file");

  //determine MIME type based on fileName extension
  String extension = fileName.substring(fileName.indexOf(".")+1);
  String mimeType;
  if (extension.equals("xls")) {
    mimeType="APPLICATION/VND.MS-EXCEL";
  }
  else if (extension.equals("doc")) {
    mimeType="APPLICATION/MSWORD";
  }
  else if (extension.equals("ppt")) {
    mimeType="APPLICATION/VND.MS-POWERPOINT";
  }
  else if (extension.equals("tiff")) {
    mimeType="IMAGE/TIFF";
  }
  else {
    mimeType="APPLICATION/STREAM";    
  }
  File downloadFile = new File(filePath+fileName);
  try {
      if (downloadFile.exists()) {
        //set response headers to force download
        response.setContentType(mimeType);
        response.setHeader("Content-Disposition","attachment; filename=\""+ fileName + "\"");
         
        //stream out file
        FileInputStream fInputStream =new FileInputStream(filePath+fileName);
        ServletOutputStream fOutputStream = response.getOutputStream();
        int i;
        while ((i=fInputStream.read()) != -1) {
            fOutputStream.write(i);
        }
        fInputStream.close();
        fOutputStream.close();
      }
      else {
            System.out.println("File not found @"+filePath+fileName);
      }
  } catch (Exception exp){
     System.out.println("Exception: "+exp.getMessage());
  }
%>