Phoenix File Uploads

Use this skill before implementing ANY file upload functionality.

Canonical FP bar: [docs/fcis-engineering-rules.md](../../docs/fcis-engineering-rules.md) — Functional Core, Imperative Shell: pure domain modules; side effects at edges. Keep LiveView/controller callbacks thin; delegate business rules to contexts/pure modules.

RULES — Follow these with no exceptions

1. Use manual uploads (NOT autoupload: true) for form submission patterns 2. Always add upload directory to staticpaths() — files won't be accessible without this 3. Handle upload errors — display errortostring/1 output in templates 4. Validate file types server-side — never trust client MIME types 5. Restart server after changing static_paths() — changes don't apply until restart

Implementation Workflow

Follow these steps in order:

1. Add uploads to static_paths() — see Static Paths Configuration below
2. Restart the server — static_paths() changes require a full restart to take effect
3. Add allow_upload/3 in mount/3 — configure accepted types, entry limits, and file size
4. Implement handle_event("validate", ...) — required to trigger change tracking
5. Implement handle_event("save", ...) — consume entries, create directories, copy files
6. Add the upload form to your template — include drop target, previews, and error display
7. Verify — confirm upload directory exists, files are saved, and URLs resolve correctly

Upload Configuration

Manual Upload (Recommended)

allow_upload(:upload_name,
  accept: ~w(.jpg .jpeg .png .pdf),
  max_entries: 10,
  max_file_size: 10_000_000
)

Complete Upload Pattern

LiveView Module

@impl true
def mount(_params, _session, socket) do
  socket =
    socket
    |> assign(:uploaded_files, [])
    |> allow_upload(:photos,
         accept: ~w(.jpg .jpeg .png),
         max_entries: 5,
         max_file_size: 10_000_000
       )

  {:ok, socket}
end

@impl true
def handle_event("validate", _params, socket) do
  {:noreply, socket}
end

@impl true
def handle_event("save", _params, socket) do
  uploaded_files =
    consume_uploaded_entries(socket, :photos, fn %{path: path}, entry ->
      dest = Path.join(["priv", "static", "uploads", safe_filename(entry.client_name)])
      File.mkdir_p!(Path.dirname(dest))
      File.cp!(path, dest)
      {:ok, ~s(/uploads/#{Path.basename(dest)})}
    end)

  {:noreply, assign(socket, :uploaded_files, uploaded_files)}
end

defp safe_filename(original_name) do
  ext = Path.extname(original_name)
  "#{Ecto.UUID.generate()}#{ext}"
end

Template

<.form for={%{}} phx-submit="save" phx-change="validate" id="upload-form">
  <div phx-drop-target={@uploads.photos.ref}>
    <.live_file_input upload={@uploads.photos} />
  </div>

  <%= for entry <- @uploads.photos.entries do %>
    <div>
      <.live_img_preview entry={entry} />
      <progress value={entry.progress} max="100"><%= entry.progress %>%</progress>

      <%= for err <- upload_errors(@uploads.photos, entry) do %>
        <p class="error"><%= error_to_string(err) %></p>
      <% end %>
    </div>
  <% end %>

  <button type="submit">Upload</button>
</.form>

def error_to_string(:too_large), do: "Too large"
def error_to_string(:too_many_files), do: "Too many files"
def error_to_string(:not_accepted), do: "Unacceptable file type"

Static Paths Configuration

# lib/my_app_web endpoint.ex
def static_paths do
  ~w(assets fonts images favicon.ico robots.txt uploads)
end

Common Pitfalls

Integration

Companion skills:

• phoenix-liveview-essentials — LiveView lifecycle and form binding
• security-essentials — validating and safely serving user-supplied files
• testing-essentials — exercising uploads with Phoenix.LiveViewTest