I tried MyBase2XBook converter. It works with two exceptions:
1. Attachments to an item are not reserved --- e.g. if I have a note.zip attached to an item, the note.zip is not in converted xbook.
2. If a MyBase item only contains attachments (i.e. empty on note itself), e.g. containing a.zip, b.zip, c.pdf, then after conversion, the item is not in xbook.