discourse/app/controllers/uploads_controller.rb

class UploadsController < ApplicationController
  before_filter :ensure_logged_in, except: [:show]
  skip_before_filter :check_xhr, only: [:show]

  def create
    file = params[:file] || params[:files].first

    # check if the extension is allowed
    unless SiteSetting.authorized_upload?(file)
      text = I18n.t("upload.unauthorized", authorized_extensions: SiteSetting.authorized_extensions.gsub("|", ", "))
      return render status: 415, text: text
    end

    # check the file size (note: this might also be done in the web server)
    filesize = File.size(file.tempfile)
    type = SiteSetting.authorized_image?(file) ? "image" : "attachment"
    max_size_kb = SiteSetting.send("max_#{type}_size_kb").kilobytes
    return render status: 413, text: I18n.t("upload.#{type}s.too_large", max_size_kb: max_size_kb) if filesize > max_size_kb

    upload = Upload.create_for(current_user.id, file, filesize)

    render_serialized(upload, UploadSerializer, root: false)

  rescue FastImage::ImageFetchFailure
    render status: 422, text: I18n.t("upload.images.fetch_failure")
  rescue FastImage::UnknownImageType
    render status: 422, text: I18n.t("upload.images.unknown_image_type")
  rescue FastImage::SizeNotFound
    render status: 422, text: I18n.t("upload.images.size_not_found")
  end

  def show
    RailsMultisite::ConnectionManagement.with_connection(params[:site]) do |db|

      return render nothing: true, status: 404 unless Discourse.store.internal?

      id = params[:id].to_i
      url = request.fullpath

      # the "url" parameter is here to prevent people from scanning the uploads using the id
      upload = Upload.where(id: id, url: url).first

      return render nothing: true, status: 404 unless upload

      send_file(Discourse.store.path_for(upload), filename: upload.original_filename)

    end
  end

end