For all types of download content always have "application/x-download" content type in header.

For PDF should be "application/pdf".

The easiest way to fix is to add parameter to download url in ExportPageAction:

• is:

      private String prepareDownloadPath(String path) throws IOException
      {
          String homeDir = new File(getBootstrapManager().getConfluenceHome()).getCanonicalPath();
          String canonicalPath = new File(path).getCanonicalPath();
          int homeDirIndex = canonicalPath.indexOf(homeDir);
  
          if (homeDirIndex != -1)
          {
              path = canonicalPath.substring(homeDirIndex + homeDir.length() + 1);
          }
  
          return "/download/" + path;
      }
  

• should be:

      private String prepareDownloadPath(String path) throws IOException
      {
          String homeDir = new File(getBootstrapManager().getConfluenceHome()).getCanonicalPath();
          String canonicalPath = new File(path).getCanonicalPath();
          int homeDirIndex = canonicalPath.indexOf(homeDir);
  
          if (homeDirIndex != -1)
          {
              path = canonicalPath.substring(homeDirIndex + homeDir.length() + 1);
          }
  
          path = "/download/" + path;
          
          if(ImportExportManager.TYPE_PDF.equals(type)){
             path += "?contentType=application/pdf";
          }
          //...more types here
  
          return path;
      }