Language specific files.encoding isn't always used

[haudan]

Issue Type: Bug

When using a language specific files.encoding, VSCode doesn't always use this encoding to open files.

I have a settings.json that contains this config (Baan is a programming language; custom extension):

"[baan]": {
    "files.encoding": "windows1252"
}

If I manually start VSCode - not opening any files or workspaces, just opening an empty instance - and then open a Baan file, that file is loaded in the correct encoding, Windows-1252.

If I start VSCode with a Baan file directly (double klicking on a source file, or passing a file path on the command line), the opened file is loaded in the default encoding, UTF-8, which is incorrect.

If I open any other file or workspace with VSCode and then open a Baan file, the encoding is correct again, Windows-1252.

---

I could reproduce this issue on a coworkers machine with his VSCode and with a different language (C in this case). The behavior is exactly the same. The only extension we both share is the Microsoft C# extension, so I assume this problem can be reproduced without any extensions.

VS Code version: Code 1.47.2 (17299e4, 2020-07-16T14:30:29.392Z)
OS version: Windows_NT ia32 10.0.18362

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-6600U CPU @ 2.60GHz (4 x 2808)
GPU Status	2d_canvas: enabled flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on oop_rasterization: disabled_off protected_video_decode: unavailable_off rasterization: enabled skia_renderer: disabled_off_ok video_decode: enabled viz_display_compositor: enabled_on viz_hit_test_surface_layer: disabled_off_ok webgl: enabled webgl2: enabled
Load (avg)	undefined
Memory (System)	7.86GB (1.70GB free)
Process Argv
Screen Reader	no
VM	0%

Extensions (32)

Extension	Author (truncated)	Version
language-x86-64-assembly	13x	2.3.0
Bookmarks	ale	11.3.1
theme-verdandi	be5	2.0.1
better-toml	bun	0.3.2
baan	dan	1.0.0
xml	Dot	2.5.1
vscode-wasm	dts	1.3.1
vsc-material-theme	Equ	32.8.0
vsc-material-theme-icons	equ	1.2.0
plantuml	jeb	2.13.12
theme-karyfoundation-themes	kar	20.0.3
rust-analyzer	mat	0.2.248
Lisp	mat	0.1.10
vscode-language-pack-de	MS-	1.47.3
remote-containers	ms-	0.128.0
remote-ssh	ms-	0.51.0
remote-ssh-edit	ms-	0.51.0
remote-ssh-explorer	ms-	0.51.0
remote-wsl	ms-	0.44.4
vscode-remote-extensionpack	ms-	0.20.0
hexeditor	ms-	1.1.0
vsliveshare	ms-	1.0.2478
debugger-for-chrome	msj	4.12.9
rust	rus	0.7.8
shader	sle	1.1.4
vba	spe	1.6.2
code-ca65	tlg	1.2.1
tmlanguage	Tog	0.9.0
vscode-todo-highlight	way	1.0.4
better-align	wwm	1.1.6
clang-format	xav	1.9.0
material-theme	zhu	3.8.5

(14 theme extensions excluded)

[bpasero]

/duplicate #39084