Markdown files starting with a HTML tag do not get rendered on Windows

Hi, thanks for the addon.

It does not render this file: https://github.com/snabbdom/snabbdom/blob/master/README.md when downloaded and then opened from within the file manager.

I guess that is because the file is huge. Smaller files get rendered without problems.

BTW, the [Gitlab Markdown viewer](https://addons.mozilla.org/de/firefox/addon/gitlab-markdown-viewer/?src=search) correctly renders it.

Reproduce the issue on Windows by downloading snabbdom's README.md and opening it from the file manager, then compare it with a smaller Markdown file. Investigate the rendering path for Markdown files beginning with an HTML tag; done means the referenced README.md renders correctly when opened locally.

Written by the indexing model from the issue text.