mu4e: use mu4e-attachment-dir for attaching, too

Use mu4e-attachment-dir also for attaching (and not just for extracting)
attachments; the default (current draft message dir) is not very useful.

NEWS.org

@@ -93,7 +93,10 @@
  ~plist~ to ~t~. For speed-reasons, these counts do _not_ filter out duplicates
       or messages that have been removed from the filesystem behind mu4e's back.
 
-    - The mu4e <-> mu intreaction has been rewritten to communicate using
+    - ~mu4e-attachment-dir~ now also applies to composing messages; it determines
+      the default directory for inclusion.
+
+    - The mu4e <-> mu interaction has been rewritten to communicate using
       s-expressions, with a repl for testing. If you have readline installed,
       you also get history.
 

mu4e/mu4e-compose.el

@@ -477,6 +477,11 @@ buffers; lets remap its faces so it uses the ones for mu4e."
           (visual-line-mode t))
       (setq mml-enable-flowed nil))
 
+    ;; set the attachment dir to something more reasonable than the draft
+    ;; directory.
+    (setq default-directory (mu4e~get-attachment-dir))
+
+
     (let ((keymap (lookup-key message-mode-map [menu-bar text])))
       (when keymap
         (define-key-after

mu4e/mu4e-vars.el

@@ -167,10 +167,15 @@ better with e.g. offlineimap."
   :safe 'booleanp)
 
 (defcustom mu4e-attachment-dir (expand-file-name "~/")
-  "Default directory for saving attachments.
+  "Default directory for attaching and saving attachments.
+
 This can be either a string (a file system path), or a function
 that takes a filename and the mime-type as arguments, and returns
-the attachment dir. See Info node `(mu4e) Attachments' for details."
+the attachment dir. See Info node `(mu4e) Attachments' for
+details.
+
+When this called for composing a message, both filename and
+mime-type are nill."
   :type 'directory
   :group 'mu4e
   :safe 'stringp)

mu4e/mu4e.texi

@@ -1340,9 +1340,9 @@ OS X) the @t{open} program for opening attachments. If you want to use another
 program, you do so by setting the @t{MU_PLAY_PROGRAM} environment variable to
 the program to be used.
 
-The default directory for extracting (saving) attachments is your home
-directory (@file{~/}); you can change this using the variable
-@code{mu4e-attachment-dir}, for example:
+The default directory for attaching and extracting (saving)
+attachmentsis your home directory (@file{~/}); you can change this
+using the variable @code{mu4e-attachment-dir}, for example:
 
 @lisp
 (setq mu4e-attachment-dir "~/Downloads")