src/vs/platform/protocol/electron-main/protocolMainService.ts/ProtocolMainService/handleFileRequest
class ProtocolMainService:	private handleFileRequest(request: Electron.ProtocolRequest, callback: ProtocolCallback): void {
		const fileUri = URI.parse(request.url);

		// isPreferringBrowserCodeLoad: false
		if (!isPreferringBrowserCodeLoad) {

			// first check by validRoots
			if (this.validRoots.findSubstr(fileUri)) {
				return callback({
					path: fileUri.fsPath
				});
			}

			// then check by validExtensions
			if (this.validExtensions.has(extname(fileUri))) {
				return callback({
					path: fileUri.fsPath
				});
			}

			// finally block to load the resource
			this.logService.error(`${Schemas.file}: Refused to load resource ${fileUri.fsPath} from ${Schemas.file}: protocol (original URL: ${request.url})`);

			return callback({ error: -3 /* ABORTED */ });
		}

		// isPreferringBrowserCodeLoad: true
		// => block any file request
		else {
			this.logService.error(`Refused to load resource ${fileUri.fsPath} from ${Schemas.file}: protocol (original URL: ${request.url})`);

			return callback({ error: -3 /* ABORTED */ });
		}
	}