I think text/html is the correct mimetype for that file. The problem for a podcast player it's the content, it's a normal webpage for user to read not the format proposed on the spec to be parsed programmatically.
https://github.com/Podcastindex-org/podcast-namespace/blob/main/docs/tags/transcript.md
https://github.com/Podcastindex-org/podcast-namespace/blob/main/docs/examples/transcripts/transcripts.md#html
:pci: Dreb Scott :pci: (npub1kga…9vap) Dave (npub1rcn…dw4h)