Just throwing this out there after looking in the filter area and see the pictures of the same filter / duct area above, my thought is now to leave the fan set to 5 or less. My thought (and I could be totally wrong) is that on a 5'ish or lower fan speed it allows the water to fall normally and drain out, but on a faster fan setting is more likely to throw the condensation in to the filters getting them nice and wet. Of course when it's 95F and sunny and you get in a hot car you really want a faster fan speed....
I think the sequence of items in the airflow path is: Inlet grille -> Fan -> Filters -> Evaporator.
Some moisture comes from outside, but I think the majority of the moisture that results in the smell is the condensation that forms on the evaporator. Keeping the condensation to a minimum is done by 1) Maximizing air flow, i.e. higher fan speeds, and 2) not making the evaporator too cold, i.e. don't set temperature too low, avoid recirc unless it's necessary, and if using recirc, use high fan speeds only. Avoid low fan speeds with recirc, as this is when the evaporator gets coldest and will condense a lot of water.
One of the biggest problems that Tesla's software has for this is that when the system is run on AUTO, it always will configure the A/C to use recirc with low fan speeds, as this is the maximum cooling for the least battery power. This is also what causes maximum condensation and maximum mildew.
Letting the car sit parked with recirc engaged is another problem -- the water then never dries because it never leaves the vehicle. Change the system to fresh air a few minutes before arriving at your destination, this will warm up the evaporator to reduce further condensation, and will allow moisture in the air to escape the car while it's parked.
It's the stupidest thing ever that this car, with all of it's software and automatic systems, fails miserably to control the HVAC system.
And he's going to make Full Self Driving work. Yeah, right.