@types/activestorage が最新に追従出来てないので雑対応

activestorage.d.ts

declare module '@rails/activestorage' {
  export function start(): void

  export class DirectUpload {
    id: number
    file: File
    url: string

    constructor(file: File, url: string, serviceName: string, attachmentName: string, delegate?: DirectUploadDelegate)

    create(callback: (error: Error | null, blob: Blob | null) => void): void
  }

  export interface DirectUploadDelegate {
    directUploadWillCreateBlobWithXHR?: ((xhr: XMLHttpRequest) => void) | undefined
    directUploadWillStoreFileWithXHR?: ((xhr: XMLHttpRequest) => void) | undefined
  }

  export interface Blob {
    filename: string
    content_type: string
    byte_size: number
    checksum: string

    id: number
    signed_id: string
    key: string
    service_name: string
    created_at: string
  }
}