Can't save as image with unusual extension

I would like to save some temporal files with extension `.temp` but I get `No known saver for '1.temp'. (Vips::Error)`

Thing like this would be nice:

```ruby
def write_to_file name, type = nil, opts = {}
  ...
  saver = Vips::vips_foreign_find_save(type || filename)
```
```ruby
im.write_to_file "1.temp", "png"
```