On upstream inclusion of the assumed_charset patch (past 1.5.14), file_charset
got renamed to attach_charset. We add it back for compatibility.

--- a/init.h
+++ b/init.h
@@ -208,6 +208,9 @@
   ** Note: for Japanese users, ``iso-2022-*'' must be put at the head
   ** of the value as shown above if included.
   */
+  { "file_charset",	DT_SYN,  R_NONE, UL "attach_charset", 0 },
+  /*
+  */
   { "attach_format",	DT_STR,  R_NONE, UL &AttachFormat, UL "%u%D%I %t%4n %T%.40d%> [%.7m/%.10M, %.6e%?C?, %C?, %s] " },
   /*
   ** .pp