Allow indicating the content-type to return in the VFS API

Currently the VFS API use Tika to try to find a content-type to set in the response but:

• Tika might be wrong
• Tika never set the encoding and unfortunately by default the application server set one which might or might not be the right one depending on the file encoding (and might break it)

Since it's not always easy to accurately find out the complete content-type for text files it should be possible to provider it in the URL.