Fixing BitmapEncoder.addFileExtension handling of uppercase extensions

[mccartney]

What: Fixes #852 by ensuring lowercase file extension is enforced, even if the original file name provided had the extension.

Also by the chance, changing how the artificial edge case of .png or .bmp as the input file name is handled.

Why:

• To satisfy the #852 issue.
• It looks like it was the intent of the code in the first place.