Forward the content_type that is passed to Convert() to the Watson API

[aaaton]

The API seems capable of detecting the content type itself, but is forced to do so every time, since w.CreateFormFile() sets the content type to application/octet-stream.

This broke the msword conversion for me, as my actual Mime-type was something like application/x-tika-msword and not application/msword.

With this change the content_type you send in to Convert() is actually passed to the Watson API.

[CLAassistant]

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