Using arrays with functions taking ranges

[Mu]

Thank you, monarch_dodra. This looks like what I wanted.

I have a question:
How come the function works with MmFile.opSlice's without 
appender(&)?
And is this reliable behavior?

caesarCipher(cast(ubyte[]) inputFile.opSlice, cast(ubyte[]) 
outputFile.opSlice, 13);

Trying to use appender(&) on the casted opSlice's yields a "is 
not an lvalue" error.