Upper-case extension does not work under Windows

[thermikfuchs]

Under Windows, in the case <base_image_path>, <image_to_compare_path>, or <output_png_path>contains the file extension in upper-case, an error message is output and no output file is generated.

odiff-windows-x64.exe before.PNG afterPNG output.PNG
odiff: internal error, uncaught exception:
       Failure("This format is not supported: .PNG")

In contrast to this, the program works perfectly with lower-case extensions.