ActiveStorage is a core Rails feature used to upload images and files. Even the most basic basic real-world apps tend to do have some form of image/file upload, e.g. user avatar.

Currently, ActiveStorage is not compatible with Mongoid (it requires ActiveRecord). This is an obstacle for many ActiveRecord users to migrate to Mongoid.

It would be great if the Mongoid team could put ActiveStorage support on the roadmap. The work to support it is likely fairly minimal, i.e. replacing AR models/generators with Mongoid fields. It may require submitting a patch to Rails. Note that typically the actual file data is stored on cloud storage like Amazon S3, and the database just contains URLs/references to the cloud data. (In other words, file data is not stored as a blob in the DB).

This has been requested previously by another user here MONGOID-4623 and was unfortunately closed.