While working on my upcoming texture-pack, I noticed that the terrain-files "Fen" and "Fen2" were the ones which contained the textures used in the "Mixed Woodlands" aka "Witche's Hat Forest".
I find this...odd to say the least, but it's good to know!
Other texture-names that don't match the generally accepted names:
-"Brush" is what we know as Badlands or Desert.
-"Leaf" is what we call Autumnal Woods.
-"Oldblack" seems to be Coniferous forest.
Less surprising names are:
-"Stone": Pavement/Cobblestone.
-"Mountain": our Stone Quarry
-"Gravel": that dark stuff you can see some places in Boston