Extract uploaded file mimetype reader

It's a one-liner, but it'll be reused and it's sort of confusing and
easy to forget or introduce a typo
Centralize common file upload spec behavior
Transmogrify asset to mimetype-based class
Extract asset file attachment into service
Add unit tests for AssetFileAttacher
Extract mimetype-to-class lookup
Add simpler AssetFileAttacher.call API
Fix unused variable initialization
Rename and namespaceify AssetFileAttacher service
Merge pull request #148 from OregonDigital/feature/mimetype-to-class

Feature/mimetype to class