If I had to guess, that cap is the cathode bypass. The cathode resistor has only a couple of volts across it.
Mylar is one type of film capacitor. Film capacitors use some type of plastic as the dielectric. Polypropylene, polystyrene, mylar, polyester...
Considering everything else going on in a tube amp, I think any of us would be hard-pressed to discern the difference between capacitor types in that location if we performed a true double-blind test. It's a fair bet that the vast majority of these "Mojo Gurus" don't even know what a double-blind test is.