How to download a file from bmc using redfish schema

LogEntry resources can provide an "AdditionalDataURI" property, which is an abstraction allowing the server to attach arbitrary data to a log entry. So, if you want to tarball the contents of /var/log and expose that through the LogEntry resource with that property, you can certainly do that.

There's also the "CollectDiagnosticData" action on the LogService, which allows the client to invoke a request to collect debug information. The result is like above; a new LogEntry is created with "AdditionalDataURI" pointing to where the client can download the requested debug information. The service can run whatever scripts or tools it wants in the background to package the debug info.