Busi4:
I was not successful with the NULL-terminated char array. The "value" i've got in the Serial-Monitor was a newLine.
The NULL terminators usually don't get stored in the files, though they -can- be.
The terminator is used in memory as an end of string marker. In file you have end-of-line which maybe anything but is usually CarriageReturn (13) or NewLine (10) or both. Check the lower-right of Serial Monitor, there are options to have it add those to every line you send from Serial Monitor to the Arduino. All these things are just marks for code to know where different ends are.
Memory --
char buffer[ 12 ] = { "string" };
// starting with address buffer the bytes are
// 115 ('s') 116 ('t') 114 ('r') 105 ('i') 110 ('n') 103 ('g') 0 0 0 0 0 0
// when it reaches 0, the loop processing the characters exits because 0 is FALSE
// the function loop reads and processes while character is TRUE, non-zero
File --
There are more different markers for files but generally it is the function that returns NULL at end of file rather than a data byte.