Hey @edsiper,
If you read our Process for reviewing classic confinement snaps, access to arbitrary files on the system isn’t typically a justification for classic.
The interfaces suggested by @ogra should help with your needs. Otherwise, could you please post here the locations of the files Fluent bit needs to access? If you still experience issues, you can try snappy-debug while troubleshooting. It will alert you of any denials and recommend missing interfaces. Feel free to post any further question and we are happy to help.