Sorry, I forgot to mention.
If you look inside the course folder (i.e. the folder, where *.wcl file is located), you can find there also folders named "courseimages", 1, 2, 3 and so on (depending on quantity of modules in your course). The "courseimages" folder contains some common files for all modules. Folders that named after numbers are modules folders (they are named automatically).
Each module folder also has standard structure: it has "images" folder, that contains all module images, sound, video etc. which was inserted in CourseLab as regular objects. Thumbnails folder is useless for author - it's design info. Author can create folder, named "user" at this level (i.e. the same with "images" and "thumbnails"). If such folder exists, then CourseLab treats it as kind of user-defined file storage and will publish whole this folder "as is" (no checks for links, just as is). This is not hidden, but poorly documented feature.
Sorry, I forgot to mention.
If you look inside the course folder (i.e. the folder, where *.wcl file is located), you can find there also folders named "courseimages", 1, 2, 3 and so on (depending on quantity of modules in your course). The "courseimages" folder contains some common files for all modules. Folders that named after numbers are modules folders (they are named automatically).
Each module folder also has standard structure: it has "images" folder, that contains all module images, sound, video etc. which was inserted in CourseLab as regular objects. Thumbnails folder is useless for author - it's design info. Author can create folder, named "user" at this level (i.e. the same with "images" and "thumbnails"). If such folder exists, then CourseLab treats it as kind of user-defined file storage and will publish whole this folder "as is" (no checks for links, just as is). This is not hidden, but poorly documented feature.