Assume you are a Druid 3 / Ranger 5.
Can you attack twice with the Flame Blade when you use Extra Attack?
The rub is that it describes it as a melee spell attack, vs. a melee attack. As such its in the same category as Inflict Wounds and other touch spells as a distinct action vs. a regular Attack action.
Extra Attack only triggers when you make the Attack action
Even if you interpret melee spell attacks as being one and the same as melee attacks, the simple fact remains that Extra Attack only triggers when the Attack action is taken.