message: previews cache control
diff: @@ -77,6 +77,8 @@ exports.init = ( app, options ) => {
  res.sendFile( previewBufferOrFile.file )
  } else {
  res.contentType( 'image/png' )
  + // If the preview is a buffer, it comes from the DB and can be cached on clients
  + res.set( 'Cache-Control', 'private, max-age=604800' )
  res.send( previewBufferOrFile.buffer )
  }
  }
  