Allow upload of doc/pptx

[tdraier]

File upload currently only allows text of pdf, we could use our tika parser to enable other upload types.

As conversion would be done on the server, this would require adding a simple entrypoint to convert files.

[tdraier]

As discussed IRL with @Fraggle and @flvndvd this is on hold until we discuss about the upload/create doc UI and the possible changes - currently uploading a document allows the user to edit and modify the content. As the conversion for pptx/docs is done on the server, this implies sending the file to the server and getting it back on the browser. Although it's perfectly doable we'd rather wait until this UI part is clarified to avoid refactoring.