[PATCH] emacs: hint that notmuch-mua-attachment-regexp is a regular expression

[Daniel Kahn Gillmor]

Without this change, we see the following warnings during the build:

```
In toplevel form:
emacs/notmuch-mua.el:119:1:Warning: defcustom for
    ‘notmuch-mua-attachment-regexp’ fails to specify type
emacs/notmuch-mua.el:119:1:Warning: defcustom for
    ‘notmuch-mua-attachment-regexp’ fails to specify type
EMACS emacs/notmuch-show.elc
```
---
 emacs/notmuch-mua.el | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/emacs/notmuch-mua.el b/emacs/notmuch-mua.el
index 94fa19d7..b1a31034 100644
--- a/emacs/notmuch-mua.el
+++ b/emacs/notmuch-mua.el
@@ -121,7 +121,8 @@ multiple parts get a header."
   "Message body text indicating that an attachment is expected.
 
 This is not used unless `notmuch-mua-attachment-check' is added
-to `notmuch-mua-send-hook'.")
+to `notmuch-mua-send-hook'."
+  :type '(regexp))
 
 ;;
 
-- 
2.20.1