Fetch locally stored file with Javascript

Question

My browser will display HTML files stored on my computer, and allow them to link to local stylesheets/scripts. It will also display text files. However, it seems like I can't use the Fetch API to access these. Is there any workaround for this?

This is what I have:

fetch('file.md')
    .then(response => response.text()
    .then(text => {
        let target = document.getElementsByClassName('content')[0];

        let converter = new showdown.Converter();

        converter.setOption('customizedHeaderId', 'true');
        converter.setOption('tables', 'true');

        let html = converter.makeHtml(text);
        target.innerHTML = html;
    })

It works fine accessing a normal Internet file.