Small cleanups in markdown drop/paste (#189985)

- Observer cancellation more places
- Create constants for mime types
- Making code more consistent to show duplication

extensions/markdown-language-features/src/languageFeatures/copyFiles/shared.ts

@@ -6,10 +6,11 @@
 import * as path from 'path';
 import * as vscode from 'vscode';
 import * as URI from 'vscode-uri';
-import { Schemes } from '../../util/schemes';
-import { NewFilePathGenerator } from './copyFiles';
 import { coalesce } from '../../util/arrays';
 import { getDocumentDir } from '../../util/document';
+import { mediaMimes } from '../../util/mimes';
+import { Schemes } from '../../util/schemes';
+import { NewFilePathGenerator } from './copyFiles';
 
 enum MediaKind {
 	Image,
@@ -49,19 +50,6 @@ export const mediaFileExtensions = new Map<string, MediaKind>([
 	['wav', MediaKind.Audio],
 ]);
 
-export const mediaMimes = new Set([
-	'image/bmp',
-	'image/gif',
-	'image/jpeg',
-	'image/png',
-	'image/webp',
-	'video/mp4',
-	'video/ogg',
-	'audio/mpeg',
-	'audio/aac',
-	'audio/x-wav',
-]);
-
 const smartPasteRegexes = [
 	{ regex: /(\[[^\[\]]*](?:\([^\(\)]*\)|\[[^\[\]]*]))/g }, // In a Markdown link
 	{ regex: /^```[\s\S]*?```$/gm }, // In a backtick fenced code block