It has been seen that during the generation of a new task it is not possible to attach files to it.
This functionality is not natively developed in Hubspot, so it is not possible in any way to attach files to tasks that are going to be registered in the CRM.
When generating the task:
The option is not presented since it is not developed in the CRM.
In case you need to attach any file to a task, you will need to create a new note containing those files, and through the mentions, notify the task manager in the note as well.