Attachments are used to link binary files or url to any openerp document.

    External attachment storage
    ---------------------------

    The computed field ``datas`` is implemented using ``_file_read``,
    ``_file_write`` and ``_file_delete``, which can be overridden to implement
    other storage engines. Such methods should check for other location pseudo
    uri (example: hdfs://hadoopserver).

    The default implementation is the file:dirname location that stores files
    on the local filesystem using name based on their sha1 hash
    