It is possible to load all channels for a team and also fetch the filesFolder property for each channel in a single request?
https://graph.microsoft.com/v1.0/teams/<id>/channels?$expand=filesFolder
is returning:
The query specified in the URI is not valid. Query option 'Expand' is not allowed. To allow it, set the 'AllowedQueryOptions' property on EnableQueryAttribute or QueryValidationSettings