transcode filetype change

[Former user]

I've got transcode working well. (thanks!). However most of my files are in mkv format. When transcoding play can't start until the entire file is transcoded which could take quite a long time. So I'm wondering if there is a way to make it so that minidlna can say the file is some other format more appropriate for streaming. I've tried having ffmpg just output another format but this seems to confuse the client and it won't play the file.