Revert r36816 for the moment (Add $wgAdditionalXMLTypes, an array of XML mimetypes...

Revert r36816 for the moment (Add $wgAdditionalXMLTypes, an array of XML mimetypes we can check for with MimeMagic.)

I very much like the idea of making this extensible, but the current implementation has a couple problems. I'd recommend addresses the following:
* The format of the array isn't documented; it has neither examples nor a description of the content format in its comment. If I wanted to add something to it, I wouldn't know what the result should look like without looking up the code.
* Rather than "additional" types, it might be best to simply list *all* the types we recognize in the default array -- then it can be modified and extended in local configuration. This would have the following benefits:
** Allows modifying existing types
** Defaults are an example of format, making the structure self-documenting
** Avoids code duplication -- we only have to check one array, not two, and don't have to worry about their formats getting out of sync.