I rarely add tasks to a project from an email as I am usually within Asana working.
But today I had to forward an email to a project. I did so and boom the task was created.
Because of signature blocks in the chain of emails, they all got added as attachments. I wanted to delete them all as I tidy up the task and I noticed that I can only delete them one file at a time.
This is both from the task itself or from the files section of the project.
There is no way to multi select and action on documents.
So my suggestion would be to introduce the ability to multi select documents for efficiency.