The update action via POST in the infostore module is extended by the following URL parameter:

offset – Optionally sets the start offset in bytes where to append the data to the document, must be equal to the actual document's length (available with v7.8.1). Only available if the underlying File storage account supports the RANDOM_FILE_ACCESS capability.

Further details are available at:

http://oxpedia.org/wiki/index.php?title=HTTP_API#Update_an_infoitem_via_PO ST