Release Notes:

OmniFocus 2 for Mac

Release Notes: What's new, updated, or fixed

OmniFocus 2.9 introduces a new attachment storage and sync system. To migrate to the new system you’ll need to be running OmniFocus 2.19 or later on all of your iOS devices and OmniFocus 2.9 or later on all of your Macs. Once all of your syncing devices are running a version that supports the new system, you’ll be prompted to migrate your database.