Index HTML files for use with lunr.js

I am busy setting up search functionality for my site using lunr.js. Searching works, but I would like to also index HTML files that I display via a shortcode.

Here is the content of my index.json file:

[{{ range $index, $value := where .Site.Pages ".Params.exclude_search" "!=" true }}
{{ if $index }},{{ end }}
  "url": "{{ .Permalink }}",
  "title": "{{ .Title }}",
  "body": {{ .Plain | plainify | jsonify }},
  "excerpt": {{ .Summary | plainify | jsonify }},
  "htmlbody": {{ .Params.htmlfile | plainify | jsonify }}
{{ end }}]

Some of my pages are using a shortcode that references an HTML file which is then displayed. I added a variable called htmlfile to the front matter of a page that embeds an HTML file, which I then reference inside the index.json file.

At the moment (obviously), the content of htmlbody is the reference to the HTML file, rather than the content of that file. How would I go about parsing the content of the referenced HTML file and assigning that content to htmlbody?

Thank you.