783 cooperate with owl api

Fixes #783.

This is kind of a dirty solution, but I think the real problem is in the OWL tools (that send different http request accept headers) for the same file download. Anyway, I fixed it in the 783 branch and merged it to develop.ontohub.org. The import link is now being created for those cases.

As soon as the owl tools developers change something in this behaviour, we must change it as well.

I'd rather simply check if we **don't** get "text/html" in the accept header (for sending the plain file), but they even use that format together with image formats.