No recognition of `mail.markdown` filetype

**Describe the bug**

I have my NeoVim configured to have Markdown injected into Mail, since I write all my emails in Markdown for post-processing. As a consequence, I set the filetype for a mail message to `mail.markdown`.

If I do so, then markdown-preview won't load the key bindings. The command `:MarkdownPreview` still works, but `cp` does not.

I have tried adding `mail.markdown` verbatim to `g:mkdp_filetypes`, but to no avail. In any case, since for Vim, `mail.markdown` is really `mail` & `markdown`, markdown-preview should load in its default configuration for filetype `mail.markdown`. That would be ideal.

**To Reproduce**

My setup is here:

The main file is `.config/lazyvim/after/ftplugin/mail` as it changes the filetype.

In my setup, `cp` is not available.

However, I could not reproduce this problem with `+"set ft=mail.markdown"`` or using a modeline.

**Expected behavior**

If I list `markdown` in `g:mkdp_filetypes`, then I expect markdown-preview to work if the filetype is `mail.markdown`

**Desktop (please complete the following information):**

- OS: Debian sid, Neovim 0.10.4-8, latest Lazyvim, latest markdown-preview