If I piped that .docx file into bas64, would it then be a "plain file"? It's ascii plaintext chars.
I don't think your argument holds any water.
I don't think your argument holds any water.
By transforming file Foo.docx into file Bar.b64 you get a plain-text file (Bar.b64) but Foo.docx still isn't plain-text. That's actually how email attachments work (transforming any file into b64 plain-text file), so I think your counter-argument is pointless.