When I am in the editor for eg. a blog post, or even this support ticket creator, I have the option to attach an image and then embed that image using the "Embed image from attachment" button.
If that image filename contains a space, then it will not render as it incorrectly generates markup text for the image as:

[[img src=File with space.png width=50%]]

The correct markup syntax is of course:

[[img src="File with space.png" width=50%]]

Solution may be one or more of:
1. Fix the forge
2. Update docu here https://forge.codesys.com/forge/wiki/Project%20Maintenance/ under the heading "Embedding Images"
3. Report bug with upstream project