Allow for uppercase file extensions

[tslouf-umich]

Noticed that I was unable to process a file with an uppercase extension despite it being a DNG. Once I changed it to ".dng" it worked as expected.

[Alex-Vasile]

Should probably change the check at the beginning of the function to also ensure that it accepts if some files are named with .dng and some with .DNG.

[martin-marek]

Looks great, thanks! I'll still keep the requirement that all extensions are the same (ie all lower- or uppercase) to prevent user error.