To remove an attached file from the attachments library:
In a template, draft or new proposal, press Attachments.
Press Add attachment.
Delete the file by pressing the trash can next to it.
The file will now be deleted from the library.
If the template was used in your templates:
If the attachment is used in any template, you'll have to remove it from each template. You do this by editing the templates and removing the file under Attachments using the red X-mark.