@jeremyruston. Sorry for tagging you. I don’t think anyone else know this better than you.
I am using flex card macro from tiddlywiki.com in my wiki to create a grid of cards as storyriver (also by using muuri plug in).
I want to use a tiddler with local png file in text field [img[relative-path.png]] in the imageField of the flex-card macro. But it was not working. Only tiddlers with image/png or image/jpeg type was working with this macro
Here is a demo wiki - https://muuri-flex-card-grid-layout-with-buttons.tiddlyhost.com/
Here is the relevant tiddler - tiddler with image field , image tiddler


