[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!

Start at the DefaultCacheManager().getSingleFile entry point described in the issue and trace how the cache filename and extension are selected from the MIME type. Add an optional extension setting that controls the returned cached file path, then verify that the resulting path uses the requested extension regardless of MIME type.

索引モデルが issue の本文から書いたものです。