Registry and dispatch for variant transformers.

Each transformer declares the (input_content_type, output_content_type) pairs it accepts via accept?/2. Attached.Variants dispatches by matching the original's content type against the variant's declared mime_type: — an image transformer covers image/ → image/, a hypothetical attached_ffmpeg could register audio/mpeg → audio/ogg, a custom module can register application/pdf → text/plain.

Ships with:

• Attached.Processors.Transformers.Image.Vix — libvips via vix (default, image/*)
• Attached.Processors.Transformers.Image.ImageMagick — convert/magick CLI (image/*)
• Attached.Processors.Transformers.Document.Pandoc — pandoc CLI (epub/docx/html/... → text/markdown)

Configuration

Transformers are tried in order. First accept?(input, output) == true wins:

config :attached, transformers: [
  Attached.Processors.Transformers.Image.Vix,
  AttachedFFmpeg.AudioTranscoder,
  MyApp.Transformers.PdfToText
]

Supported transforms (built-in image transformers)

Both Vix and ImageMagick support the same transform keys:

Non-image transformers

Custom transformers can target any content-type pair. Example (PDF → plain text) — registered in config as above:

defmodule MyApp.Transformers.PdfToText do
  @behaviour Attached.Processors.Transformers.Behaviour

  @impl true
  def accept?("application/pdf", "text/plain"), do: true
  def accept?(_, _), do: false

  @impl true
  def transform(input_path, _transforms, output_path) do
    case System.cmd("pdftotext", [input_path, output_path]) do
      {_, 0} -> :ok
      {out, code} -> {:error, {:exit, code, out}}
    end
  end
end