Can not display local image (ver. 2.0.2.8)

Can't display local image file. Don't know which is last working version.

I tried relative path (with correct file location), but nothing to display. (Online image url working)

```md
```

Also, can't click ``view Source`` on addon's menu.

## System

* OS: Debian GNU/Linux 11 (bullseye)
* Firefox: 102.12.0esr（64 bit）
* Addon: Markdown Viewer Webext 2.0.2.8

Another addon include:
* NoScript
* uBlock Origin
(But I don't see they block anything. ``NoScript`` show can't change authority, but I still try to click allow all in this page.)

No source files or tests are named. First reproduce the problem in Markdown Viewer WebExt 2.0.2.8 using the three relative-image examples on Debian/Firefox, then investigate the add-on menu's View Source action. Done means local relative images display while online images continue to work, and View Source is clickable.

Written by the indexing model from the issue text.