You can now migrate files stored under selected Libraries. This can be done in two ways:
- Real-time migration using the Migrate screen

- Batch migration by submitting a batch

PS: View Mass File Migrate Salesforce file migration app on Salesforce AppExchange:
https://appexchange.salesforce.com/appxListingDetail?listingId=e9874a66-2001-4e3d-8a29-3a2ed7f584c4