Save attached files when

C code was mostly copy pasted from
$MUPDF/platform/gl/gl-annotate.c:save_attachment_dialog

This piggybacks on existing savepath-command.

Testing was done when savepath-command was set to "echo '%s'" and a
document from https://github.com/moosotc/llpp/issues/170 [1]. The
annotation rectangles an the fifth page overlap making it impossible
to use mouse to click on the icon with a pin (file attachment
annotation), however one can enter keyboard navigation mode (set to
insert by default) and select last/covered annotation rectangle with
the keyboard) (selecting this annotation via hint mode (bound to 'F'
by default) works too)

[1] https://github.com/moosotc/llpp/files/6566150/CorrectionLEMAIRE_SAKURA.AMELIE_CPFR_DEV02.pdf

ffi.ml

@@ -43,6 +43,8 @@ external hasunsavedchanges : unit -> bool = "ml_hasunsavedchanges"
 external savedoc : string -> unit = "ml_savedoc"
 external gettextannot : opaque -> slinkindex -> string = "ml_gettextannot"
 external getfileannot : opaque -> slinkindex -> string = "ml_getfileannot"
+external savefileannot : opaque -> slinkindex -> string -> unit
+  = "ml_savefileannot"
 external wcmd : Unix.file_descr -> bytes -> int -> unit = "ml_wcmd"
 external rcmd : Unix.file_descr -> string = "ml_rcmd"
 external uritolocation : string -> (pageno * float * float) = "ml_uritolocation"

link.c

@@ -2522,6 +2522,20 @@ ML (getfileannot (value ptr_v, value n_v))
     CAMLreturn (ret_v);
 }
 
+ML0 (savefileannot (value ptr_v, value n_v, value path_v))
+{
+    CAMLparam3 (ptr_v, n_v, path_v);
+    struct page *page = parse_pointer (__func__, String_val (ptr_v));
+
+    lock (__func__);
+    struct slink *slink = &page->slinks[Int_val (n_v)];
+    pdf_obj *fs = pdf_dict_get (state.ctx, slink->u.annot->obj, PDF_NAME (FS));
+    fz_buffer *buf = pdf_load_embedded_file (state.ctx, fs);
+    fz_save_buffer (state.ctx, buf, String_val (path_v));
+    fz_drop_buffer (state.ctx, buf);
+    unlock (__func__);
+}
+
 ML (getlinkrect (value ptr_v, value n_v))
 {
     CAMLparam2 (ptr_v, n_v);

main.ml

@@ -2760,8 +2760,21 @@ let gotounder = function
      let pageno, x, y = Ffi.uritolocation s in
      addnav ();
      gotopagexy pageno x y
-  | Utext _ | Unone  | Ufileannot _ -> ()
+  | Utext _ | Unone -> ()
   | Utextannot (opaque, slinkindex) -> enterannotmode opaque slinkindex
+  | Ufileannot (opaque, slinkindex) ->
+     if emptystr conf.savecmd
+     then adderrmsg "savepath-command is empty"
+            "don't know where to save attachment"
+     else
+       let filename = Ffi.getfileannot opaque slinkindex in
+       let savecmd = Str.global_replace Re.percent filename conf.savecmd in
+       let path =
+         getcmdoutput
+           (adderrfmt savecmd
+              "failed to obtain path to the saved attachment: %s") savecmd
+       in
+       Ffi.savefileannot opaque slinkindex path
 
 let gotooutline (_, _, kind) =
   match kind with