Simply drag and drop files into a task to attach them
Alternatively, you can:
Click on the Task to open its Task Properties.
Click on Attach Files to add files to the task in the Comments and Activity section.
You can also add files directly from Google Drive, Dropbox and Box.
Hit the Enter key to add files to your task.
In order to view all the files and links attached to a task, click on Files & Links tab in Task Properties.