code: @IntRange(from = -1)
      long getExactContentLengthRangeFrom0(@NonNull DownloadConnection.Connected connected) {
          final String contentRangeField = connected.getResponseHeaderField(CONTENT_RANGE);
          long contentLength = -1;
          if (!Util.isEmpty(contentRangeField)) {
              final long rightRange = getRangeRightFromContentRange(contentRangeField);
              // for the range from 0, the contentLength is just right-range +1.
              if (rightRange > 0) contentLength = rightRange + 1;
          }
  
          if (contentLength < 0) {
              // content-length
              final String contentLengthField = connected.getResponseHeaderField(CONTENT_LENGTH);
              if (!Util.isEmpty(contentLengthField)) {
                  contentLength = Long.parseLong(contentLengthField);
              }
          }
  
          return contentLength;
      }
docs: Get the exactly content-length, on this method we assume the range is from 0.