Cannot link to a file from the WYSYWIG editor that was previously uploaded into lessons
From e-Learning Documentation
Because Lessons is not a core tool and was developed seperately it has its own file repository. Core tools such as Discussions and Assignments use Resources as their repository.
Solution: Upload a second version of the file to Resources to be used in Assignments & Discussions or anywhere you can link to the file from the HTML WYSIWYG editor.