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.
This creates a new type of action in addition to the 9 or so enumerated in the PHB, if you use it you are not using the Attack action upon which all multiple attack features rely.
You cast a spell when you summon the blade in to existence.
Melee "spell" attack is in reference to what mod you can use.
If you chose to use your Wisdom mod treat it as a spell attack.
Then the answer would be no, you do not.
If you use your dexterity or strength mod like you would a normal sword swing
Then you are going thru a physical attack motion and that should trigger any
thing swinging a normal "flaming scimitar" would.
Then the answer would be yes, you get your second attack.
Note, there is nothing in the spell description baring you from making normal attacks with your brand new flaming scimitar.