Store all the uploaded files.

This function is expected to be called inside a transaction:

Multi.new()
|> Multi.insert(:user, user_changeset)
|> Multi.run(:upload_files, fn _repo, %{user: user} ->
  Uploader.store_files(user)
end)
|> Repo.transaction()
|> case do
  {:ok, %{user: user}} ->
    {:ok, user}

  {:error, :user, %Ecto.Changeset{} = changeset, _changes} ->
    {:error, changeset}

  {:error, :upload_files, {:file_path_exists, file_path}, _changes} ->
    raise "file upload failed: path \"#{file_path}\" already exists"
end

This function fetches all the uploadable schema fields (with all of their options) from the given Schema struct (see Uploader.Ecto.UploadableFields), and for each field calls the maybe_copy_file/4 callback function. This callback function is called from the module defined in the :uploader option of the field (by default, the module Uploader.LocalUploader is used). Below is an example on how to define a custom module implementing the maybe_copy_file/4 callback function in the Schema:

schema "users" do
  uploadable_field :avatar_image, uploader: AmazonS3Uploader
  # other fields
end

In case of success {:ok, nil} is returned, otherwise {:error, error_tuple} is returned with error_tuple being a two-elements tuple describing the error. This tuple is in fact the returned error value of the call to the maybe_copy_file/4 function. An example of a returned error value is {:error, {:file_path_exists, file_path}}.