When you are creating a new Planner task, there’s a reasonable chance that you may want to add a relevant file to the task. Now Microsoft provides a list of suggested files at the bottom of the task when you’re setting it up. Here’s what that looks like…
Here’s a task that I created:

When I scroll to the bottom, I’m given the option to select from a list of selected attachments that may be relevant to this task. I could click on any one of these items, and the attachment would be added to the task:

The list of attachments is generated via machine learning based on the task and the user. The more you use it, the better the suggestions will become over time.