email: support MIME Content-ID header

- Reads from and writes into the Body parameter list.

email/parse.c

@@ -1379,6 +1379,23 @@ struct Body *mutt_read_mime_header(FILE *fp, bool digest)
         mutt_str_replace(&p->description, c);
         rfc2047_decode(&p->description);
       }
+      else if (mutt_istr_equal("id", line + plen))
+      {
+        // strip <angle braces> from Content-ID: header
+        char *id = c;
+        int cid_len = mutt_str_len(c);
+        if (cid_len > 2)
+        {
+          if (id[0] == '<')
+          {
+            id++;
+            cid_len--;
+          }
+          if (id[cid_len - 1] == '>')
+            id[cid_len - 1] = '\0';
+        }
+        mutt_param_set(&p->parameter, "content-id", id);
+      }
     }
 #ifdef SUN_ATTACHMENT
     else if ((plen = mutt_istr_startswith(line, "x-sun-")))

send/header.c

@@ -767,6 +767,8 @@ int mutt_write_mime_header(struct Body *a, FILE *fp, struct ConfigSubset *sub)
   int tmplen;
   char buf[256] = { 0 };
 
+  char *id = NULL;
+
   fprintf(fp, "Content-Type: %s/%s", TYPE(a), a->subtype);
 
   if (!TAILQ_EMPTY(&a->parameter))
@@ -784,6 +786,13 @@ int mutt_write_mime_header(struct Body *a, FILE *fp, struct ConfigSubset *sub)
       struct Parameter *cont = NULL;
       TAILQ_FOREACH(cont, &pl_conts, entries)
       {
+        if (mutt_istr_equal(cont->attribute, "content-id"))
+        {
+          // Content-ID: gets its own header
+          id = mutt_str_dup(cont->value);
+          break;
+        }
+
         fputc(';', fp);
 
         buf[0] = 0;
@@ -816,6 +825,12 @@ int mutt_write_mime_header(struct Body *a, FILE *fp, struct ConfigSubset *sub)
 
   fputc('\n', fp);
 
+  if (id)
+  {
+    fprintf(fp, "Content-ID: <%s>\n", id);
+    mutt_mem_free(&id);
+  }
+
   if (a->language)
     fprintf(fp, "Content-Language: %s\n", a->language);