Files with a mime type that does not match the extension are now properly thumbnailed...

Files with a mime type that does not match the extension are now properly thumbnailed. Partial bug fix to bug 26661.
MediaHandler::getThumbType now checks if the extension and the mime type match. If they do not, a new extension is generated for the mime type. The rest of the thumbnailing code should then magically work.
Bonus: this also works with file names without extension, so might be a step closer to extension-less files.