FileUtil: Refactor detectMediaType()

Make detectMediaType returns MediaType instead of String. MediaType is
easier than String to get useful information from itself.
CodeBrowser: Add missed charset for raw view

Add charset parameter to the Content-Type for the resource requested by
clicking Raw button on Code Browser.
FileUtil: Comment why we use mozilla's charset detector
Merge branch 'fix/missed-charset-for-raw-view' of npcode/yobi

from pull request 1023