If you run the following:
mkvextract attachments file.mkv 1:1.ttf 2:2.ttf 3:3.ttf 4:4.ttf 5:5.ttf 6:6.ttf ... etc
Then mkvextract will extract attachments by id until it can't find an attachment. It will exit when it fails to find an attachment, so the batch will progress. Disadvantage here is that you lost the attachment filenames, but without grepping around some mkvinfo output, this is the only way I know of to extract all the attachments.