It only support for now adding, modifying and getting file.
indexstorage is needed to get the list of documents.

They were useless because native method Array.prototype.indexOf,
Math.min and Math.max do the same thing.

If you put {
  "_id": "a.b",
  "_attachment": "compressed.tar.gz",
  "_data": ...
}, the backend will write "a_.b.compressed.tar.gz"
instead of "a_.b.compressed_.tar_.gz"