Register custom element just once

The file-attachment-element module is responsible for exporting the
class while the index module registers it in the browser's custom
element registry.

Author: dgraham

src/file-attachment-element.ts

@@ -1,6 +1,6 @@
 import Attachment from './attachment'
 
-class FileAttachmentElement extends HTMLElement {
+export default class FileAttachmentElement extends HTMLElement {
   constructor() {
     super()
     this.addEventListener('dragenter', onDragenter)
@@ -48,13 +48,6 @@ class FileAttachmentElement extends HTMLElement {
   }
 }
 
-export default FileAttachmentElement
-
-if (!window.customElements.get('file-attachment')) {
-  window.FileAttachmentElement = FileAttachmentElement
-  window.customElements.define('file-attachment', FileAttachmentElement)
-}
-
 function hasFile(transfer: DataTransfer): boolean {
   return Array.from(transfer.types).indexOf('Files') >= 0
 }