Replace file extension by another file extension
file_subs_ext(fn, new_ext, check = TRUE)Returns a character vector of the same length of fn with the
extension of the file names in fn replaced by new_ext.
character vector with file names
character vector of length one with the new extension (when it does not start with a period a period is added).
check if the new file name is not equal to the original filename. If so, generate an error.