I'm not sure Movemybooks will be able to convert data with attachments. As you may have noticed, they made a tool to convert only core data from QB Desktop to QB Online without any manual work. https://movemybooks.co.uk/move-to-quickbooks-old/limitations-of-service-quickbooks/

We have converted our client data from QB Online to other accounting apps and the attachment is one of the components we have to convert half manually.