more type safety

Author: Feiyang1

packages/storage/src/implementation/string.ts

@@ -22,7 +22,7 @@ import { decodeBase64 } from '../platform/base64';
  * An enumeration of the possible string formats for upload.
  * @public
  */
-export type StringFormat = string;
+export type StringFormat = typeof StringFormat[keyof typeof StringFormat];
 /**
  * An enumeration of the possible string formats for upload.
  * @public
@@ -60,7 +60,7 @@ export const StringFormat = {
    * be overridden in the metadata object).
    */
   DATA_URL: 'data_url'
-};
+} as const;
 
 export class StringData {
   contentType: string | null;