Bugged extensions when using Saver with custom filename

[physinet]

Currently, if you provide Saver with a custom filename ending in .json, for example, then Saver generates a .json.json file., which it then has problems loading.

[physinet]

There is also a warning message for saving over a file that already exists that only appears for the weird case where Saver tries to create a .json.json file. I meant for this to appear for the normal case.