We use the native content rendering component supplied as part of the iOS operating system in the app. If .eml files aren't being rendered then I'm afraid you'll have to raise an issue or improvement with Apple and hope they incorporate support in future versions of iOS.
No. There's nothing you can configure. It's simply not supported by Apple. The supported formats listed in the developer documentation are: <ul> <li>iWork documents <li>Microsoft Office documents (Office ‘97 and newer) <li>Rich Text Format (RTF) documents <li>PDF files <li>Images <li>Text files <li>Comma-separated value (csv) files </ul>