Fix undef extension

**Bugfix:** If an `options` object is passed without an 'ext' attribute the extension will be undefined when the file is written. Commit 0824f801c018d486c42e1d36609056c1a2cf0c42 fixes it.

The next two commits (fe6c5b20601143d8879a68f6c886c6a9b8fbf8f5 & a5a28f4262cd3dbbf9aea4f7b0c778d153169e9b) are not strictly related, feel free to skip them if it doesn't conform to your guidelines.

The last commit (8d283e0ba7675b2e78781ba49072da0b75163c18) bumps the patch version