If we can't determine the file type, use the generic
'application/octet-stream' type. This way `file.type` will always be
defined and uploaders don't have to worry about it being `null` or
whatever.
Available as `@uppy/utils/lib/Translator`.
The Translator tests were using `Core()` instead of `Translator()`, so I
changed it to using `Translator` instances directly. The functionality
being tested did not change.
Steps taken:
```bash
git mv src/core/Translator.* packages/@uppy/utils/src
sed -i 's/[./]*\/core\/Translator/@uppy\/utils\/lib\/Translator/' src/**/*.js
vim packages/@uppy/utils/src/Translator.test.js # Fix tests
```
