Correctly fetch file name and other values

Fields like the core upload field use `ExportableField::VALUE` to return plain values but don't offer any output for `ExportableField::UNFORMATTED`. SBL should check both modes.

Related to symphonycms/symphony-2/#1805
Merge pull request #45 from nilshoerrmann/integration

Correctly fetch file name and other values using the Exportable interface