Godot: Add engine.js function to override file name extensions

[KylaCpper]

**Godot version:2.14||3.0

**OS/device including version:html

Issue description:
Hi everyone, I am a godot developer
In my country (China)
Want to release a html5 game to the platform
The required suffix name format must be the following
htm,html,js,css,jpg,jpeg,gif,png,mp3,ogg,m4a,xml,csv,dat,atf,json,plist,wav,thm,fnt,exportjson,ttf,tmx,exml,datagz,jsgz,memgz。
However, godt 2.1 comes packaged with .pck and .mem
3.0 packaged with .pck and .wasm

I tried changing mem's suffix name to .dat and then loading .mem in index.js and index.html. I've changed it. But once I change .pck, it won't work.

Change .wasm to another extension such as .dat in index.js .wasm to .dat can also be run
But not pck

Is there any way to not use pck file or replace it

Steps to reproduce:

Minimal reproduction project:

[KylaCpper]

Pck file name changed after running, reported error（3.0）

[leonkrause]

First of all, some workarounds:

With 2.1.4 or earlier:

• in index.html, change index.mem to e.g. index.mem.dat and rename index.mem accordingly.

• in indexfs.js, change only the first occurrence of data.pck to e.g. data.pck.dat and rename data.pck accordingly, e.g.:

  fetchRemotePackage('data.pck.dat', function(data) {

With 2.1.5 or newer, godotfs.js is gone, so:

• in index.html, (just like in 2.1.4,) change index.mem to e.g. index.mem.dat and rename index.mem accordingly

• also in index.html, change only the second occurrence of data.pck to e.g. data.pck.dat and rename data.pck accordingly, e.g.:

  Module.FS.createPreloadedFile('/data.pck', null, 'data.pck.dat', true, true, null, null, false, true);

With 3.0:

• in index.js, change '.wasm' to the desired file name extension, e.g.:

  engineLoadPromise = loadPromise(basePath + '.wasm.dat').then(function(xhr) {

• a custom .pck can be loaded using the standard API in the .html file, change:

  engine.startGame(BASENAME + '.pck').then(() => {

  to:

  const MAIN_PACK = BASENAME + '.pck.dat';
  Promise.all([Engine.load(BASENAME), engine.preloadFile(MAIN_PACK)]).then(() => {
  engine.start('--main-pack', MAIN_PACK);
  }).then(() => {

[leonkrause]

I understand that .pck can be an issue since it's a custom Godot format. That's why I added a Promise-based JavaScript API in 3.0 to give more control, like in the example above where we manually load the file and specify it as the main pack per argv.

But .wasm is a standard format, web game portals need to add support for this. WebAssembly is quickly becoming the default target for web games. The WebAssembly.instantiateStreaming() function, used for fast start-up, in fact requires a correct MIME-type, so having a correct file name extension is vital.

Can you say what web game platform this is? Does it have a large user-base?

[KylaCpper]

@eska014
Thank you very much
I do as you say ，Succeeded

You solved my big problem

The platform I am talking about is 4399 （www.4399.com）and Don't know if you can access

This platform is mainly for small games ，It used to be a platform for flash games，Profit from advertising，
There are many Chinese people and the proportion of netizens is high，Income is good
Actually, I originally wanted to release steam，but steam to pay 100 US dollars
4399 is free

Sorry, my English is not good

[KylaCpper]

Most of the people around me are using (u3d, cocos, etc.)
Almost nobody uses Godot, I hope more and more people will know Godot.
There are many users in China and the market is very large.
I used unity3d before and found that the godot works well
If you can come to China for publicity,
Absolute effect is very good

[leonkrause]

You're welcome, glad I could help.

This is a legitimate issue, so I'll add an API to override file name extensions, at least for 3.0