uploads_controller.rb

class Api::UploadsController < ApiController

  def create
    @upload = Upload.new(upload_params)
  ensure
    clean_tempfile
  end

  private

  def upload_params
    the_params = params.require(:upload).permit(:file)
    the_params[:file] = parse_image_data(the_params[:file]) if the_params[:file]
    the_params
  end

  def parse_image_data(base64_image)
    filename = "upload-image"
    in_content_type, encoding, string = base64_image.split(/[:;,]/)[1..3]

    @tempfile = Tempfile.new(filename)
    @tempfile.binmode
    @tempfile.write Base64.decode64(string)
    @tempfile.rewind

    # for security we want the actual content type, not just what was passed in
    content_type = `file --mime -b #{@tempfile.path}`.split(";")[0]

    # we will also add the extension ourselves based on the above
    # if it's not gif/jpeg/png, it will fail the validation in the upload model
    extension = content_type.match(/gif|jpeg|png/).to_s
    filename += ".#{extension}" if extension

    ActionDispatch::Http::UploadedFile.new({
      tempfile: @tempfile,
      content_type: content_type,
      filename: filename
    })
  end

  def clean_tempfile
    if @tempfile
      @tempfile.close
      @tempfile.unlink
    end
  end
end