Power Apps allows attachments to be added to entities in CDS via a canvas app. However, attachments can't be added or accessed via a model-driven app using the same CDS. Microsoft released a Power Apps hands on lab a year ago that walks through creating a front-end canvas app for users that integrates with a back office model-driven app through CDS for Apps. This is a very useful and powerful scenario. The training does not include demonstrating the use of attachments. If it had, you would see that the only way for the back office folks to get to the attachments is to create another canvas app for them; which would be confusing as they would need to access both the canvas app and the model-driven app to get their work done.