I've found the FreeRTOS +FAT with optional support of long filenames (http://www.freertos.org/FreeRTOS-Plus/FreeRTOSPlusFAT/index.html) so that was my trigger to start evaluating it on my Nordic NRF52 eval kit. I use Eclipse & GCC on a Windows 7 64-bit machine to evaulate the FreeRTOS version 9.0.0 FAT functionality. A Micro-SD card holder has been hand-soldered (including pull-up resistors) onto some GPIO pins that are configured for SPI use on the Nordic platform.

The RAM driver example worked so I used that to create an (micro-) SD card driver over SPI, also working fine now. However, any file I create has uppercase characters when viewed in Windows explorer and I cannot seem to use filenames longer than 8 characters + 3 character extension. I can live with uppercase filenames but really would like to use longer filenames than this. I found that increasing FMAXNAME to 100 characters in fatsl.h does not bring a solution.

So the main question is: How do I enable the 'optional' support for long filenames ?

I think you might be confusing two different file systems.

FreeRTOS+FAT has optional long file name support: http://www.freertos.org/fat

but fatsl.h is not a FreeRTOS+FAT header file, it is a "FreeRTOS+FAT SL" header file: http://www.freertos.org/fatsl

The SL stands for 'Super Lean', and is limited in functionality. Despite its similar name, the code base has nothing in common with the full FAT file system implementation as its a completely different code base.