Some background info:

Flash Size: 4 MB
SPIFFS Flash: 1.5MB
Uploaded SPIFF .bin size: 1.3MB
I use MKSPIFFS to generate the .bin file using the following:

-b 4096
-s 0x13D620
-p 256
i.e. 4096 block, 1.3mb, 256 page

I have been able to fetch a SPIFFS .bin file with HttpUpdate updateSPIFFS (from ...