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.