It would be interesting to know if anybody still uses the HasManyFileManager module?
If yes, you have probably thought about updating to DataObjectManager?

I'm currently in the process of migrating several Sites from SilverStripe 2.2.x or 2.3.x to version 2.4.2. Since most of these Sites used the HasManyFileManager module, I had to write a migration script that moves my HasManyFileManager relations to DataObjects.

I wrote a recipe how you can perform such a migration/upgrade if you're still running an old SilverStripe Version with HasManyFileManager:
http://doc.silverstripe.org/recipes:migratefilemanager

The migration script I wrote is also linked in the recipe above. I guess there are also tons of grammatical and spelling errors in that recipe, so please feel free to correct them.