Annotate return types to fix tsc warnings

Author: dgraham

src/attachment.ts

@@ -43,15 +43,15 @@ export default class Attachment {
     return ['image/gif', 'image/png', 'image/jpg', 'image/jpeg'].indexOf(this.file.type) > -1
   }
 
-  saving(percent: number) {
+  saving(percent: number): void {
     if (this.state !== 'pending' && this.state !== 'saving') {
       throw new Error(`Unexpected transition from ${this.state} to saving`)
     }
     this.state = 'saving'
     this.percent = percent
   }
 
-  saved(attributes?: {id?: string | null; href?: string | null; name?: string | null}) {
+  saved(attributes?: {id?: string | null; href?: string | null; name?: string | null}): void {
     if (this.state !== 'pending' && this.state !== 'saving') {
       throw new Error(`Unexpected transition from ${this.state} to saved`)
     }

src/file-attachment-element.ts

@@ -23,7 +23,7 @@ export default class FileAttachmentElement extends HTMLElement {
     }
   }
 
-  async attach(transferred: File[] | Attachment[] | FileList | DataTransfer) {
+  async attach(transferred: File[] | Attachment[] | FileList | DataTransfer): Promise<void> {
     const attachments =
       transferred instanceof DataTransfer
         ? await Attachment.traverse(transferred, this.directory)