Exports from version 4.x are not compatible with FAL.
On importing such an export in version 6.x the images needs
to be moved and added to sys_file.
- related images from uploads/ (e.g. tt_content:images and pages:media) are moved to fileadmin/_import/.
- linked files are moved to the original folder structure below fileadmin/_import/.
- (!!!) Files from fields of old type group file should NOT be moved