The behaviour of a file store for attachments.

Kotoba.Live.consume_uploads/4 stores each uploaded file through the configured adapter, and puts the URL that the adapter returns in the attachment node:

config :kotoba, storage: Kotoba.Storage.Local

The default adapter is Kotoba.Storage.Local.

Keys

A key is a relative path with segments of ASCII letters, digits, ., - and _, separated by /. A segment is never . or ... key/2 makes a new key in the form yyyy/mm/<uuid>-<safe name><extension>. The extension comes from the checked content type (Kotoba.Attachments.extension/1), never from the client's file name, so a key never ends in .html or .svg. An adapter refuses a key that valid_key?/1 refuses.

Serving the files

An uploaded file is content from a user. Serve it with X-Content-Type-Options: nosniff, with its checked content type, and with Content-Disposition: attachment for every type that is not an image, a PDF or a video (see Kotoba.Attachments.inline?/1). Kotoba.Storage.Local.Plug does this for the local adapter; the example below does it for S3.

Another store

To keep the files somewhere else (S3, another object store, a database), write a module with the three callbacks and name it in the config, or give it per call with the :storage option of Kotoba.Live.consume_uploads/4. For example, with ExAws.S3:

defmodule MyApp.KotobaStorage do
  @behaviour Kotoba.Storage

  @bucket "my-app-uploads"

  @impl true
  def put(key, path, meta) do
    disposition =
      if Kotoba.Attachments.inline?(meta.content_type), do: "inline", else: "attachment"

    path
    |> ExAws.S3.Upload.stream_file()
    |> ExAws.S3.upload(@bucket, key,
      content_type: meta.content_type,
      content_disposition: disposition
    )
    |> ExAws.request()
    |> case do
      {:ok, _response} -> {:ok, url(key)}
      {:error, reason} -> {:error, reason}
    end
  end

  @impl true
  def url(key), do: "https://#{@bucket}.s3.amazonaws.com/" <> key

  @impl true
  def delete(key) do
    case ExAws.request(ExAws.S3.delete_object(@bucket, key)) do
      {:ok, _response} -> :ok
      {:error, reason} -> {:error, reason}
    end
  end
end

config :kotoba, storage: MyApp.KotobaStorage

The URL goes into the stored document, so it must stay valid: a public or CDN URL, or a path in the app (for example "/files/" <> key) whose controller checks access and redirects to a short-lived signed URL. The file first comes to the LiveView (a normal LiveView upload); the adapter then sends it on.