This is a patch from upstream which is likely included in the next
version (likely to be named 1.0):

From: Steve Hubert <hubert@washington.edu>
To: Alpine Alpha List
Date: Thu, 6 Sep 2007 09:58:58 -0700 (PDT)
Subject: Re: Content-Type different after postponing

I believe the attached patch should fix this. Thanks.

From: Andreas Schamanek
Date: Thu, 6 Sep 2007 20:13:47 +0200 (CEST)
I have just tested it, and it works perfectly.

From: Gerald Pfeifer

Sorry, it doesn't.  Also with this patch I am still getting

  Content-Type: TEXT/PLAIN; charset=ISO-8859-15

when sending mail directly that contains an Umlaut, and

  Content-Type: TEXT/PLAIN; CHARSET=UTF-8

after posting and resuming the same message.

From: Steve Hubert <hubert@washington.edu>

The first patch only worked if Downgrade Multipart to Text was turned on.
This should fix the other case.

Index: pith/send.c
===================================================================
--- pith/send.c	(revision 709)
+++ pith/send.c	(working copy)
@@ -843,6 +843,14 @@
 		return(redraft_cleanup(streamp, TRUE, flags));
 	    }
 		
+	    if((charset = rfc2231_get_param(part->body.parameter,"charset",NULL,NULL)) != NULL){
+		/* let outgoing routines decide on charset */
+		if(!strucmp(charset, "US-ASCII") || !strucmp(charset, "UTF-8"))
+		  set_parameter(&part->body.parameter, "charset", NULL);
+		  
+		fs_give((void **) &charset);
+	    }
+
 	    ps_global->postpone_no_flow = 1;
 	    get_body_part_text(stream, &b->nested.part->body,
 			       cont_msg, "1", 0L, pc, NULL, NULL, GBPT_NONE);