I suggest that links and files that are included in lessons should not have to be embedded in the main lesson content.
I found it more helpful when (in the past) links and files (some of which are optional reading) could merely be listed on the right side of the lesson.
For now, I will put optional content at the bottom of the lesson, but I think this is not an ideal option as it will leave the lesson looking rather messy at the bottom. It will probably also confuse some students, who, I anticipate, will ask me many times if they have to read everything at the links and files I include in the body of the lesson.