log.Print("Missing argument for files direcory, Exiting...")
os.Exit(0)
Exit code 0 is usually used for success. This is clearly an error.
filePath + "/" + file.Name()
There's path/filepath module for cross-platform joining of file paths.
Don't use defer f.Close() in the loop — defer will execute after the function returns. What you're doing is opening all of the files in the directory and then closing them only when the program exists.
if file.IsDir() == false {
Already boolean, this should be
if !file.IsDir() {
Apart from all of this, it looks like the algorithm is very inefficient with all of those slice manipulations.
