The define FG tiles should look like this.
Note that the diagonal corner tiles are defined. The handy thing about this is that it automatically fills in the appropriate corner when you Fill Visible Walls (command-F, Tiles menu).
Oh yeah about about your sorting through your world datas.
I have a "maps" folder, in which I have a folder for each world, "Default", "Andrew's World", "Basic Nothingness", etc. Convenient for sorting them. But also you need to put the name of the world in its Comments. Otherwise if you forget which world you're using you have to bother to figure out which one it is so you know where to put it.
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The only good is knowledge and the only evil is ignorance. --Socrates
Many of life's failures are people who did not realize how close they were to success when they gave up. --Thomas Edison
Lack of will power has caused more failure than lack of intelligence or ability. -- Flowers A. Newhouse