Add file upload validation policies

[SwimmingRieux]

Introduce validation rules to restrict uploads based on file size, allowed MIME types and probably other factors. This ensures only supported and safe files are accepted by the system.

Validation rules must be updated using grpc and kraken

[SwimmingRieux]

@kehiy

What policies do you have in mind, other than size and type restrictions?

[kehiy]

@SwimmingRieux, here are some of them:

1. size.
2. type.
3. author black/white list (we have to support storage limit based on pubkey, which needs a proper design. @ZigBalthazar can help with this).