Allow default uploader when using external uploaders in LiveView

[nallwhy]

In the current implementation of Phoenix.LiveView.Upload, when you configure external: &function/2, it becomes mandatory to specify an uploader.

This makes sense in most cases, but there’s an important limitation: Once you introduce even a single external uploader, you lose the ability to fall back to the default built-in uploader.

This can be problematic when you want to mix different uploader strategies depending on the file — for instance, using a custom external uploader for images, but sticking to the built-in uploader for PDFs or other files.

I’ve patched LiveView locally to support falling back to the default uploader even when external: &function/2 is set — and it works well in my testing.

defmodule MyApp.SampleLive do
  ...
  def mount(_, _, socket) do
    socket =
      socket
      |> allow_upload(:files, external: &presign_upload/2)

    {:ok, socket}
  end

  defp presign_upload(entry, socket) do
    meta =
      case entry.client_type do
        "image/" <> _ -> %{uploader: "S3", presigned_url: "..."}
        _ -> %{uploader: :default}
      end

    {:ok, meta, socket}
  end
end

If simply returning :default for the uploader in meta seems inappropriate.

https://elixirforum.com/t/allow-default-uploader-when-using-external-uploaders-in-liveview/70501

[Copilot]

Pull request overview

This PR updates Phoenix.LiveView.Upload to allow an external: &fun/2 preflight to selectively fall back to the built-in channel uploader (via {:ok, :default, socket}), enabling mixed upload strategies within a single upload configuration.

Changes:

• Extend external preflight handling to accept {:ok, :default, socket} and return a signed channel upload token for that entry.
• Refactor upload consumption to support consuming a mix of channel-uploaded and externally-uploaded entries under the same upload config.
• Add unit + E2E coverage demonstrating mixed default/channel and external uploaders.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/phoenix_live_view/upload/external_test.exs	Adds coverage for external preflight returning :default to use built-in uploader.
test/e2e/tests/uploads.spec.js	Adds E2E scenario mixing default and external uploaders in one form submission.
test/e2e/test_helper.exs	Adds a test external uploader implementation and an external upload endpoint for E2E tests.
test/e2e/support/upload_live.ex	Adds mixed-upload LiveView behavior and renders uploader type for assertions.
lib/phoenix_live_view/upload.ex	Implements :default fallback for external preflight and refactors consumption to handle mixed entries.
lib/phoenix_live_view/test/upload_client.ex	Updates test upload client to handle external entries (meta map tokens) and mixed uploads.
lib/phoenix_live_view/test/live_view_test.ex	Uses a unified upload client start path for external and channel uploads.

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