You can attach files to your tasks from:
If you are using a 3rd party service, be sure that the files you want to link are accessible by your team members (allowing them access from the selected provider).
For linking a file, go to the Task detail panel, and click the "+" button on the right side of the files section. Select the desired service and follow the steps.
To remove a file linked to a task, just click the trash icon next to it.