Slack SDK has a loose typing

FileUpload.ts

// https://github.com/slackapi/node-slack-sdk/blob/1312f6222497ca58af6788ea32744d7fd85964ce/packages/web-api/src/methods.ts#L1975-L1984

type Buffer = object;
type Stream = object;

type FileUpload = (
  | {
      channels?: string; // comma-separated list of channels
    }
  | {
      channels: string; // comma-separated list of channels
      thread_ts: string; // if specified, `channels` must be set
    }
) &
  (
    | {
        content: string; // if omitted, must provide `file`
      }
    | {
        file: Buffer | Stream | string; // if omitted, must provide `content`
      }
  ) &
  Partial<{
    filename: string;
    filetype: string;
    initial_comment: string;
    title: string;
  }>;

const u0: FileUpload = {
  content: "this is a content.",
};

const u1: FileUpload = {
  channels: "foo,bar",
  content: "this is a content.",
};

// @ts-expect-error no channels, but no error
const u2: FileUpload = {
  thread_ts: "foo",
  content: "this is a content.",
};

// @ts-expect-error no content nor file
const u3: FileUpload = {
};

const u4: FileUpload = {
  file: "this is a content.",
};

const u5: FileUpload = {
  file: "this is a content.",
  filename: "content.txt",
  filetype: "txt",
  title: "content",
};