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/util/mimes.ts

@@ -0,0 +1,22 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+export const Mime = {
+	textUriList: 'text/uri-list',
+	textPlain: 'text/plain',
+} as const;
+
+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',
+]);