TextEdit turned HTML file into RTF while open - how to undo?

I was on a Mac with no plaintext text editor installed, and needed to make a very quick change to a HTML file saved on a shared drive. I opened it in TextEdit, set it to "Plain text" in Preferences (which had no effect), hit Format > Make Plain Text, hit undo since that didn't help, then I closed TextEdit without saving.

It gave some warning about not being able to store previous versions of the file because the shared drive doesn't support that, and wouldn't quit until I chose "okay". I thought this would be fine since I hadn't saved any changes.

Somewhere along the line, TextEdit had quietly deleted the original HTML file and created an RTF version. This unwanted RTF's creation date was from around the time I opened it.

There's nothing in trash, and I've tried "Revert to" in TextEdit (no previous versions).

Is there anything I can do to get the original file back?

Operating system is 10.9.5 if that's relevant.