Don't open other file types

[Dr-Wrong-Mo]

There are some specific files that trigger the preview pane to open with this extension, even though they aren't markdown files. Specifically, I have a few PHP files that trigger this.

[SuEric]

The same it is happening to my VS Code. I found out that it happens with I used with the "Github Pull Requests" extension:

https://github.com/Microsoft/vscode-pull-request-github.git