to be future changes-proof don't rely on mime gem file extensions order

We taking first extension matching mime type while remote downloading. 
mime-types (2.3) gem has this json definition:
```json
{"content-type":"image/jpeg","encoding":"base64","extensions":["jpe","jpeg","jpg"], ....
```
So, this spec fails: 
https://github.com/carrierwaveuploader/carrierwave/blob/be840d15eaf8900f48241ab6d802fd62b5e3b584/spec/uploader/download_spec.rb#L112-L115