[BUG] externalimages build process destroy jpg file

[linonetwo]

Describe the bug
I tried running following custom "externalimages" build process on local machine:

        "externalimages": [
            "--setfield",
            "[is[image]]",
            "_canonical_uri",
            "$:/core/templates/canonical-uri-external-image",
            "text/plain",
            "--setfield",
            "[is[image]]",
            "text",
            "",
            "text/plain",
            "--rendertiddler",
            "$:/plugins/tiddlywiki/tiddlyweb/save/offline",
            "index.html",
            "text/plain"
        ],

And it makes xx.jpg 's content changed to �w^~)�

To Reproduce
Steps to reproduce the behavior:

1. Clone https://github.com/linonetwo/Meme-of-LinOnetwo/tree/beeff69eb1ba9c93e0fac24d4c0d0384a82ab88c
2. npm i
3. npm run build
4. See error in ./MemeOfLinonetwo/tiddlers/webcatalog-tiddlywiki-menu-app.jpg

Expected behavior
The build process should not change the file inside tiddlers, it should be a pure function without side effect.

Screenshots

Desktop (please complete the following information):

• OS: MacOS 10.15.4 (19E287)
• Browser: Firefox 74.0.1 (64-bit)
• Version: TW 5.2.1

Additional context
I use image uploaded to Github as src for img, so I just let generated HTML have _canonical_uri.

[KendrickLamarck]

Possible duplicate of #4484

[linonetwo]

Sorry for duplicate...