Harper doesn't work on text files?

I've got harper configured with helix, and it works really well for markdown files. However, it doesn't seem to want to work on text files (or any other non-markdown file I've tried). Is this an inherent limitation?

Helix allows specifying per-file-type language servers, so it would be really useful to be able to remove this limitation from the lsp server (if it indeed exists).

Reproduce the behavior through Helix using markdown and plain-text files, then inspect the LSP server behavior for differences between those document types. Confirm what Harper currently accepts and define done as receiving grammar diagnostics for text files and other intended non-markdown files.

Written by the indexing model from the issue text.