Cache file extension

The issue we encountered was having a file with name of the form "somefile.JPG" from an external source, which is passed through WPThumb, which returns the URL of the resized, cached image ( "the/cached/image.JPG" ).

However, WordPress is known to lowercase the extensions of uploaded files. This meant WPThumn return "the/cached/image.JPG", while the image is infact "the/cached/image.jpg".

The lowercase extension is problematic on Linux and Unix-like systems, where file names are case-sensitive.