Hi,
File attachements do not synchronize between QuickBooks Desktop and Method, this is a limitation with the current QuickBooks SDK integration.
If you are in need of accessing QuickBooks files, there are third party products avaialble (such as SmartVault) that integrate with QuickBooks. SmartValut allows you to store documents in the cloud and relate them to QuickBooks entities and transactions. Method has an integration with SmartValut as well.