jio.get({"_id": "a/b"});
changed to
jio.getAttachment({"_id": "a", "_attachment": "b"});

To test it, go to the storage directory then type: node synchro.js

// Before:
jio.putAttachment({
  "id": "docid/attmtid",
  "data": "abc",
  "mimetype": "text/plain",
  "rev": "xxxx"
});

// Now:
jio.putAttachment({
  "_id": "docid/attmtid",
  "_data": "abc",
  "_mimetype": "text/plain",
  "_rev": "xxxxx"
});