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!

webaccount/account.ts

@@ -669,6 +669,8 @@ const index = async () => {
 				ContentType: 'text/plain',
 				ContentTypeParams: {charset: 'utf-8'},
 				ContentID: '',
+				ContentDisposition: '',
+				Filename: '',
 				DecodedSize: 8,
 				Parts: [],
 			},