Tentative might be an understatement.
There was 4 browser sign-off on using that mime type within the WebAssembly CG, which has drafted the spec so far.
We're in the process of handing v1 of the spec off to the WebAssembly Working Group on our way to a good and proper W3C REC.
However, use of the mime type is already shipping in Chrome and I believe on the way with other browsers (and just got added to our shared toolchain).

That said we should get a proper allocation with IANA.
I've added an agenda item to our next Working Group meeting to have the group explicitly empower me to approach IANA:
https://github.com/WebAssembly/meetings/blob/master/2017/WG-10-23.md

Be back in month with a IANA request email to reference.

Assuming we've sorted out a proper allocation, do you have a sense of if getting the file type into python2.7 is likely?

Thanks!