It seems like attaching a file is done using the image icon (the one that in the rest of the world means "place a link to an image inside IMG / IMG tags so that it is shown as image).
Here the hover on text is “upload”, BUT it only accepts a (limited) set of image types, any different one will get you:
Sorry, the file you are trying to upload is not authorized (authorized extensions: jpg, jpeg, png, gif, heic, heif, webp, avif).
So, it doesn’t accept svg (just like the old board) but it doesn’t accept .zip or .txt (or .rsc) files.
Then, probably because I did attach a .png (with the new version of the flowchart), my post became “pending approval”, so it doesn’t show. on the thread:
Let’s hope that these are just some initial default settings that need to be tuned.