If you have only one, you have a Filofax.... but if you have more than one then what do you have?
Several Filofaxes? Filos? what is the correct spelling for the plural of Filofax?
You might have noticed I often skirt around the problem by writing 'Filofax Organisers' putting the plural on the organisers part... cheating I know, but heh I was an engineer, who had two attempts to pass his English Language O Level, so what do I know about these complicated rules of languages!!