web browsers have recently gained ability to run webassembly code and for that a new content type has to be add to web servers for optimal use:

wasm => Content-Type header : application/wasm

spec says it :
https://webassembly.github.io/spec/web-api/index.html#streaming-modules

"Firefox streaming compilation needs Content-Type header set"

cf: https://groups.google.com/forum/#!topic/emscripten-discuss/C7-i1gqWay4

google's filament documentation says:
"Python's simple server [...] does not serve WebAssembly files with the correct MIME type."

it would be logical since simple htt server is mostly used for testing software to offer support of that new techonology.

Python 3.7 is in feature-freeze, so any new features like this will have to be 3.8 only.

We don't generally add a mime type until it is a de-jure or de-facto standard.  If it is still in testing it is probably too soon to add it.  For testing, you can always add it yourself in your code via the api that mimetypes provides.

Sure, but i was considering the cpython in the browser/webview/electron case where python modules are served as .wasm files, i'm about to do it for my port and pyodide already does. 
It would make sense to provide the *simplest* environnement for cpython to serve its own components.

scientific cpython stack in the browser:
https://github.com/iodide-project/pyodide

python repl in the browser experiment:
http://pmpp.pagesperso-orange.fr/python_em.html

Also i think cpython in the browser could be be already beyond testing and so far provides the best sandboxing configuration available around :
"run python in a sandbox, not the opposite" ( quoting Victor Stinner )

According to Issue 34758, this was already added to 3.8’s “mimetypes”.