add Content-Disposition and Filename to the payload of incoming webhooks

for each message part. The ContentDisposition value is the base value without
header key/value parameters. the Filename field is the likely filename of the
part. the different email clients encode filenames differently. there is a
standard mime mechanism from rfc 2231. and there is the q/b-word-encoding from
rfc 2047. instead of letting users of the webhook api deal with those
differences, we provide just the parsed filename.

for issue #258 by morki, thanks for reporting!

webapisrv/server.go

@@ -1263,9 +1263,12 @@ func (s server) MessageGet(ctx context.Context, req webapi.MessageGetRequest) (r
 		MailboxName:         mb.Name,
 	}
 
+	structure, err := webhook.PartStructure(log, &p)
+	xcheckf(err, "parsing structure")
+
 	result := webapi.MessageGetResult{
 		Message:   msg,
-		Structure: webhook.PartStructure(&p),
+		Structure: structure,
 		Meta:      meta,
 	}
 	return result, nil