Using content-type for choosing the viewer instead of extension

[francesco-strazzullo]

Here's a duplicate of this issue #52. But much more cleaner! ;)

[kossebau]

Hi @francesco-strazzullo, thanks for the proposal and sorry for ignoring your patch for so long.
ViewerJS 0.5.5 just has been released, and it comes with a solution similar to what your PR proposes, doing an initial request before the actual to learn about the contenttype.
But other than proposed here it uses the HEAD request, which seems suited for the usecase. See webodf/WebODF#859

So closing this PR now as discarded. At least it was a little bit inspiring to what has been done now, thanks again.