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>").
 emacs/notmuch-lib.el | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/emacs/notmuch-lib.el b/emacs/notmuch-lib.el
index ece29c3..fc67b14 100644
--- a/emacs/notmuch-lib.el
+++ b/emacs/notmuch-lib.el
@@ -504,7 +504,7 @@ (defun notmuch-parts-filter-by-type (parts type)

 (defun notmuch-get-bodypart-binary (msg part process-crypto)
-  "Return the unprocessed content of PART in MSG.
+  "Return the unprocessed content of PART in MSG as a unibyte string.

 This returns the \"raw\" content of the given part after content
 transfer decoding, but with no further processing (see the
@@ -515,6 +515,16 @@ (defun notmuch-get-bodypart-binary (msg part 
                ,@(when process-crypto '("--decrypt"))
                ,(notmuch-id-to-query (plist-get msg :id)))))
+      ;; Emacs internally uses a UTF-8-like multibyte string
+      ;; representation by default (regardless of the coding system,
+      ;; which only affects how it goes from outside data to this
+      ;; internal representation).  This *almost* never matters.
+      ;; Annoyingly, it does matter if we use this data in an image
+      ;; descriptor, since Emacs will use its internal data buffer
+      ;; directly and this multibyte representation corrupts binary
+      ;; image formats.  Since the caller is asking for binary data, a
+      ;; unibyte string is a more appropriate representation anyway.
+      (set-buffer-multibyte nil)
       (let ((coding-system-for-read 'no-conversion))
        (apply #'call-process notmuch-command nil '(t nil) nil args)

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