code: public long chunkedUploadAppend(String uploadId, long uploadOffset, byte[] data, int dataOffset, int dataLength)
          throws DbxException
      {
          return chunkedUploadAppend(uploadId, uploadOffset, dataLength, new DbxStreamWriter.ByteArrayCopier(data, dataOffset, dataLength));
      }
docs: Append data to a chunked upload session.
  
  @param uploadId
  The identifier returned by {@link #chunkedUploadFirst} to identify the chunked upload
  session.
  
  @param uploadOffset
  The current number of bytes uploaded to the chunked upload session.  The server checks
  this value to make sure it is correct.  If it is correct, the contents of {@code data}
  is appended and -1 is returned.  If it is incorrect, the correct offset is returned.
  
  @param data
  The data to append.
  
  @param dataOffset
  The start offset in {@code data} to read from.
  
  @param dataLength
  The number of bytes to read from {@code data}, starting from {@code dataOffset}.
  
  @return
  If everything goes correctly, returns {@code -1}.  If the given {@code offset} didn't
  match the actual number of bytes in the chunked upload session, returns the correct
  number of bytes.