xml import/export MDL-25103 was not working due to out-of-memory errors when calling xmltidy, which broke XML wellformedness.
The solution is to not tidy the XML. The XML generated is already quite well laid out, and if we want it better, we should fix that, rather than trying to load the whole file into memory to reformat it.
Also, while investigating this, I found that we were not handling the mime-type of the exported file very elegantly, so I added a new mime_type method (defaults to getting the mime type of the file extension).
