You'll need to install a separate extension for FlatBuffers syntax highlighting. This one works.
Extension Settings
You can configure how the extension finds the language server binary.
flatbuffers.languageServer.autoDownload: Have the extension automatically download updates to the language server. (Default: false)
flatbuffers.languageServer.path: Specify a path to the language server binary. When unset, the extension will search for a binary in your PATH before offering to download one.