message: fix files can be nullable
diff: import { Embed } from "../embeds/embed.ts";
  import { AllowedMentions } from "../messages/allowed_mentions.ts";
  -import { FileContent } from "../misc/file_content.ts";
  import { Attachment } from "../messages/attachment.ts";
  +import { FileContent } from "../misc/file_content.ts";
  /** https://discord.com/developers/docs/resources/webhook#edit-webhook-message-jsonform-params */
  export interface EditWebhookMessage {
  @@ -10,7 +10,7 @@ export interface EditWebhookMessage {
  /** Embedded `rich` content */
  embeds?: Embed[] | null;
  /** The contents of the file being sent/edited */
  - file?: FileContent | FileContent[];
  + file?: FileContent | FileContent[] | null;
  /** Allowed mentions for the message */
  allowedMentions?: AllowedMentions | null;
  /** Attached files to keep */
  