Out of the box CK5 editor has new ways of dealing with files and images, and can be configured differently in Backdrop and Drupal
Backdrop CMS out of the box
File upload in CKE is handled via link plugin. So you need the link plugin, and in your text format the setting the "File upload" must be enabled (in the vertical tabs below the Editor toggle). And there's also a Filter module permission to let people "Upload files through editor dialogs" in the permission matrix. Check the roles that should be able to upload.
