We recently ran into a small bug with the DataObjectDecorator while working with the hasmanyfilemanager. We were extending the core File object to contain a few extra fields.. but for some reason the function getMyCMSFields in the decorator was not getting called in the CMS..
We found that renaming our project folder from [whatever] back to "mysite" solved the problem.
i.e. from [silverstripe root]/[whatever] to [silverstripe root]/mysite
Any ideas as to what causes this problem?