TIL Macs come with a command line base64 decoder. Somehow Mail failed to recognize that an attachment was an attachment so I copy/pasted a giant wall of base64 into that to get the expected PDF back out.
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Also that it's a lot faster to paste ~1MB of text into a text editor and save that someplace reasonable than it is to paste that same text into Terminal. I gave up waiting on that and assume I might have run into some kind of maximum line length in the shell had I kept waiting for that to finish.
