Hi!
Unfortunately that is not possible directly. You have to use a workaround, that has its limits however.
- First, you must create the task manually, using the "Create list item" action.
- Then you have to use the "Send email" action, to send task notification, where you can attach files.
- Then you have to use the "Pause for" action, to stop your workflow until the task is not completed.
But in that approach you will not see your tasks listed in the "Workflow History" page. You will also have troubles if you create more than one task at a time, etc...
Anyway, as I wrote - you cannot do it using either "Assign a task" or "Start a task process" actions. Possibly there is such request in the UserVoice 
Regards,
Tomasz