[Feature request] override extension file

I need to enforce a specific extension to the cached file whatever the MIME type of the file is.

It would be nice to have an option for that, for example:

`var file = await DefaultCacheManager().getSingleFile(url, {extension: 'my'});`

The end result with be a path with `file:///somewhere/in/the/phone/myfile.my`.

Thank you!