GraphQL - search all files that have a particular extension

Within a repository, is there a way to find for “*.rb” for example?

Not via the GraphQL search connection, no. Currently, it does not support file results. For now, you’ll have to use the REST search API.

It is possible to search for all files with a specific extension in all public repositories? Github APIs always require to specify an organization/user/repository name, but what if I want to search on all repositories without specifying name or organization?