Update LatestEdits.xslt

When displaying the thumbnail for the media, the file-extension may be a different casing than the `umbracoExtension` property.

I encountered an issue where a content editor uploaded images with the format `IMG1234.JPG`, with the `umbracoExtension` being stored as "jpg".

The `translate` function makes the filename lowercase for the thumbnail check.
Merge pull request #1 from leekelleher/patch-1

Fixes issue with casing on umbracoExtension