emacs: Return unibyte strings for binary part data

Unibyte strings are meant for representing binary data.  In practice,
using unibyte versus multibyte strings affects *almost* nothing.  It
does happen to matter if we use the binary data in an image descriptor
(which is, helpfully, not documented anywhere and getting it wrong
results in opaque errors like "Not a PNG image: <giant binary spew
that is, in fact, a PNG image>").