When you do it as post(conn, table_upload_path(conn, :create, table_upload: params)), you are passing the parameters as part of the query string, i.e. as part of the URL. You need to pass it as part of the post:

conn = post(conn, table_upload_path(conn, :create), table_upload: params)

IIRC, when I was working with file uploads in tests, I was creating Plug.Upload struct manually and passed it as regular parameter to get, post etc. macros in controller tests:

post conn, image_path(conn, :create), upload: %Plug.Upload{...}

EDIT: but of course the question is about changesets I think @OvermindDL1 is with the right approach.

All right! I have since learned that the :path field is a path to a temporary file (including the filename itself). The :filename is the name used during uploading, which can be used to do (a weak form of!) MIME-checking, and possibly to store it in your app, etc.

I was able to call ImageMagick using the extracted :path. (Yes, this code should be cleaned up and get some documentation, but it’s a proof-of-concept.)

defmodule Cmd.Magick do
  def dimensions(file_path) do
    case System.cmd("identify", ["-format","%w %h", Path.expand(file_path)]) do
      {res, 0} ->
        [width, height] =
          res
          |> String.split
          |> Enum.map(&String.to_integer/1)
        %{width: width, height: height}
      {_other, _} ->
        raise ArgumentError
    end
  end
end

In my Phoenix 1.3 changeset function I’ve added a call to this function to the pipeline:

  def add_image_dimensions(changeset, attrs) do
    case attrs["image"] do
      %Plug.Upload{path: path, filename: filename} ->
        changeset
        |> cast(Cmd.Magick.dimensions(path), [:width, :height])
      path when is_binary(path) ->
        changeset
        |> cast(Cmd.Magick.dimensions(path), [:width, :height])
      _ ->
        changeset
    end
  end

This works!

What I am yet to figure out though, is how this code can be tested properly. Please help!