A useful PR! Thanks for that work!

Looks good for me and works with our carrierwave.

Things, I would implement once this PR is merged (I dont know how to create PR which is based on another PR):

Arguments expires and options for url()
https://github.com/inkoop/fog-backblaze/blob/6-private-bucket/lib/fog/backblaze/storage/models/file.rb#L56
As in https://github.com/fog/fog-aws/blame/8185a56f25b12746669e869f72d8e9207a77aa08/lib/fog/aws/models/storage/file.rb#L240
I will implement expires and options[:b2ContentDisposition]

The "default" expiry is imo too long for a private bucket:

I also noticed, public_url should be nil, if bucket is private, as in fog-aws: https://github.com/fog/fog-aws/blame/8185a56f25b12746669e869f72d8e9207a77aa08/lib/fog/aws/models/storage/file.rb#L170

@philister Great! I hope it gets reviewed and merged soon.

Thank you so much! I will need some time to review this