FML started the lowercase enforcement in 1.11, but has been strongly advising it since 1.7 at least.
And it benefits everyone on Windows. (Or any non-Unix based systems, rather)
While in the development environment (IDE), "FileOne.png" and "fileone.png" could point to the same file. However, when you create your JAR (which is really just a ZIP file with different name) the OS cannot find "FileOne.png" any longer, if it was called "fileone.png", or "FiLeOnE.png" or whatever you called it, because of the structure of the mentioned compressed system.
As such, it would work inside your IDE, but not when you pack your mod. Countless of modders have had this issue, and reported this "bug".
And it is in no way "outrageous". Naming conventions, even for simple file IO's, makes it so much easier to organize your own files, and work with other's projects.