Add uppercased versions of image file extensions for automatic module inference
_The documentation is not available anymore as the PR was closed or merged._
Adds the uppercased versions of the image file extensions to the supported extensions. 

Another approach would be to call `.lower()` on extensions while resolving data files, but uppercased extensions are not something we want to encourage out of the box IMO unless they are commonly used (as they are in the vision domain)

Note that there is a slight discrepancy between the image file resolution and `imagefolder` as the latter calls `.lower()` on file extensions leading to some image file extensions being ignored by the resolution but not by the loader (e.g. `pNg`). Such extensions should also be discouraged, so I'm ignoring that case too.

Fix #4514.  