Hi Jiri,
As you've noticed currently FileSyncProvider has its own conflict resolution policies and does not support specifying custom ones. While technically you could set the property to something it is strongly recommended that you don't as it will mess with FSP's internal conflict resolution and may result in non-optimal handling of conflicts. We're considering adding custom/extensible conflict resolution policies to FSP but it's not something we've been able to fit into the current release. For now please don't set the property to anything. Note that the same is true in native code where the application should specify CRP_NONE in the ISyncSession.Start call if it's using native FSP providers.
thanks,
Lukasz Gwozdz