code: private String getContentId(File baseDir, File file) {
          String filePath = file.getPath();
          String basePath = baseDir.getPath();
  
          int index = filePath.indexOf(basePath);
          if (index == -1) {
              StringBuilder sb = new StringBuilder("Invalid basePath for file: ");
              sb.append("b: '" + basePath + "', ");
              sb.append("f: '" + filePath + "'");
              throw new DuraCloudRuntimeException(sb.toString());
          }
  
          String contentId = filePath.substring(index + basePath.length());
          if (contentId.startsWith(File.separator)) {
              contentId = contentId.substring(1, contentId.length());
          }
          // Replace backslash (\) with forward slash (/) for all content IDs
          contentId = contentId.replaceAll("\\\\", "/");
          return contentId;
      }
docs: This method defines the returned contentId as the path of the arg file
  minus the path of the arg baseDir, in which the file was found.
  
  @param baseDir dir that contained the arg file or one of its parents
  @param file    for which contentId is to be found
  @return contentId of arg file