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!

webmail/api.go

@@ -904,7 +904,12 @@ func (w Webmail) MessageSubmit(ctx context.Context, m SubmitMessage) {
 							ap = ap.Parts[xp]
 						}
 
-						filename := tryDecodeParam(log, ap.ContentTypeParams["name"])
+						_, filename, err := ap.DispositionFilename()
+						if err != nil && errors.Is(err, message.ErrParamEncoding) {
+							log.Debugx("parsing disposition/filename", err)
+						} else {
+							xcheckf(ctx, err, "reading disposition")
+						}
 						if filename == "" {
 							filename = "unnamed.bin"
 						}