Get the size of an image file using Nginx Embedded Lua module

gistfile1.txt

## Get the size of the image file using Lua.
## Cf. http://wiki.nginx.org/HttpLuaModule#set_by_lua.
location ~* \.(?:gif|jpe?g|png)$ {
    set_by_lua $img_file_size '
      function fsize (file)
        local current = file:seek()      -- get current position
        local size = file:seek("end")    -- get file size
        file:seek("set", current)        -- restore position
        return size
      end
      fsize(ngx.var.request_filename)';
}

That gave me error failed to run set_by_lua*: set_by_lua:3: attempt to call method 'seek' (a nil value), and I fixed by using this:

set_by_lua $file_size '
  function fsize (file_name)
    local file = assert(io.open(file_name, "r"))   -- open file
    local current = file:seek()                    -- get current position
    local size = file:seek("end")                  -- get file size
    file:seek("set", current)                      -- restore position
    assert(file:close())                           -- close file
    return size
  end
  return fsize(ngx.var.request_filename);
';