file.rb

require 'tempfile'

class UploadStore
  class File
    attr_reader :file

    # Public:
    #
    # file - Fog file.
    def initialize(file)
      @file = file
    end

    # Internal: Wraps the processing of the upload file. Ensures that upload is
    # properly cleaned up after processing.
    #
    # Returns a block result.
    def process(&blk)
      result = tempfile(&blk)

      # Should we add destroy to an ensure block? Im not sure if it makes sense
      # to delete failed files.
      file.destroy

      result
    end

    def name_parts
      parts = file.key.split('/').last.split('.')

      # Re-introduce the '.' for the file extension
      parts << ".#{parts.pop}"
    end

    def tempfile
      tmp = Tempfile.new(name_parts)

      tmp.binmode

      begin
        tmp.write(file.body)
        tmp.rewind
        yield tmp
      ensure
        tmp.close
        tmp.unlink
      end
    end

    def destroy
      file.destroy
    end
  end
end

file_functional_spec.rb

require 'spec_helper'

describe UploadStore::File do
  before do
    # This is a lot of work to set up but its very worth it to ensure that
    # this core processing logic is functioning as expected.

    connection = Fog::Storage.new({
      provider:   'Local',
      local_root: Rails.root.join('tmp'),
      endpoint:   Rails.root.join('tmp')
    })

    directory = connection.directories.create(key: 'fog-test')

    @fog_file = directory.files.create(
      :key    => 'world-bar.jpeg',
      :body   => File.open(Rails.root.join('spec/fixtures/world-bar.jpeg')),
      :public => true
    )

    @upload_file = UploadStore::File.new(@fog_file)
  end

  it 'uses the file extension with the tempfile' do
    @upload_file.process do |file|
      expect(file.path).to end_with('.jpeg')
    end
  end
end

file_unit_spec.rb

require 'unit_helper'
require 'upload_file'

# TODO: Stub Tempfile and remove the file system from the unit tests.
describe UploadStore::File do
  let(:decorated) { double(:file, destroy: true, body: 'body', key: 'key.ext') }
  let(:file) { UploadStore::File.new(decorated) }


  it 'splits name parts to preserve the file extension' do
    expect(file.name_parts).to eql(['key', '.ext'])
  end

  it 'process deletes the uploaded file' do
    decorated.should_receive(:destroy)
    file.process {|file| }
  end

  it 'process returns the value of the block' do
    val = file.process {|file| 'block_value' }
    expect(val).to eql('block_value')
  end
end