Fix missing declare and upload.file doesn't exist

- Fix ERR: A 'declare' modifier is required for a top level declaration in a .d.ts file.

- Fix ERR: Property 'file' does not exist on type 'File' 
when using it like `this.image = URL.createObjectURL(upload.file)`

- `VueUpload.File` type is conflicted with https://developer.mozilla.org/en-US/docs/Web/API/File so need to change to VUFile (ts don't have a way to resolve this)

index.d.ts

@@ -1,9 +1,9 @@
 import Vue from 'vue'
 
 // Instance / File
-global {
-  namespace VueUpload {
-    interface File {
+declare global {
+  interface VUFile {
+    file: File
     readonly fileObject: boolean
     id: string | number
     size: number
@@ -24,21 +24,20 @@ global {
     iframe: Element
   }
 }
-}
 
-class _ extends Vue {
+declare class _ extends Vue {
   // Instance / Methods
-  get(id: VueUpload.File | object | string): VueUpload.File | object | boolean
-  add(files: Array<VueUpload.File | File | object> | VueUpload.File | File | object): object | Array<VueUpload.File | object> | boolean
-  addInputFile(el: HTMLInputElement): Array<VueUpload.File>
-  addDataTransfer(dataTransfer: DataTransfer): Promise<Array<VueUpload.File>>
-  update(id: VueUpload.File | object | string, data: object): object | boolean
-  remove(id: VueUpload.File | object | string): object | boolean
-  replace(id1: VueUpload.File | object | string, id2: VueUpload.File | object | string): boolean
+  get(id: VUFile | object | string): VUFile | object | boolean
+  add(files: Array<VUFile | File | object> | VUFile | File | object): object | Array<VUFile | object> | boolean
+  addInputFile(el: HTMLInputElement): Array<VUFile>
+  addDataTransfer(dataTransfer: DataTransfer): Promise<Array<VUFile>>
+  update(id: VUFile | object | string, data: object): object | boolean
+  remove(id: VUFile | object | string): object | boolean
+  replace(id1: VUFile | object | string, id2: VUFile | object | string): boolean
   clear(): boolean
 
   // Instance / Data
-  readonly files: Array<VueUpload.File>
+  readonly files: Array<VUFile>
   readonly features: { html5?: boolean; directory?: boolean; drag?: boolean }
   active: boolean
   readonly dropActive: true