FileUpload lost after upgrading from 3.x to 4.x


We are migrating Rocketchat from the old server to the new.
After restoring mongodb dump to the db name “parties” and switched to 4.x with Snap, all fileuploads are gone with the placeholder “Retry”. The storage method I am using is GridFS which is the default.
Could somebody help me how to migrate the file uploads also?

Server Setup Information

  • Version of Rocket.Chat Server: 3.2.1 to 4.6.2
  • Operating System: CentOS 7.9
  • Deployment Method: snap
  • Number of Running Instances: 1
  • DB Replicaset Oplog:
  • NodeJS Version: 12.0.0
  • MongoDB Version: 4.0.28
  • Proxy: nginx
  • Firewalls involved: No

Any additional Information