neomutt: specify path to mime.types

Neomutt will by default look for /etc/mime.types which isn't present in order to
map file extensions to mime types.

If this file isn't found, and a message has a png attached with
application/octet-stream mime type, neomutt will not know what to do with it.

pkgs/applications/networking/mailreaders/neomutt/default.nix

@@ -42,6 +42,10 @@ in stdenv.mkDerivation rec {
         --replace http://docbook.sourceforge.net/release/xsl/current     ${docbook_xsl}/share/xml/docbook-xsl \
         --replace http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd ${docbook_xml_dtd_42}/xml/dtd/docbook/docbookx.dtd
     done
+
+    # allow neomutt to map attachments to their proper mime.types if specified wrongly
+    substituteInPlace sendlib.c \
+      --replace /etc/mime.types $out/share/doc/neomutt/mime.types
   '';
 
   configureFlags = [