Kinto-attachment

**Bug fixes**

- Add missing content type when uploading to S3

**Bug fixes**

- Fix broken compatibility with Kinto 13.6.4

**Bug fixes**

- Fix attachment deletion not being committed (fixes 149)

**Internal changes**

- Use ``unittest.mock`` instead of the ``mock`` library

**Bug fixes**

- Fix support of kinto >= 12

**Breaking changes**

- Do not allow any file extension by default. Now allow documents+images+text+data (Fix 130)
To allow more extensions (see [pyramid storage extensions configuration](https://pythonhosted.org/pyramid_storage/#configuration)), use something like `kinto.attachment.extensions = default+video`

**Bug fixes**

- Fix heartbeat when allowed file types is not ``any`` (Fix 148)

**Breaking changes**

- Gzip ``Content-Encoding`` is not used anymore when uploading on S3 (fixes 144)

**Internal changes**

- Heartbeat now uses ``utils.save_file()`` for better detection of configuration or deployment errors (fixes 146)