File's encryption tool in terminal
If you crack argv using argparse you'll enjoy a couple of benefits, including you get --help for free.

Defining read &amp; WRITE_MODE is maybe a little over the top, as open(f, 'w') is a standard idiom. Anyway, you have an opportunity to let with close files for you, if you like. For example:

    with open(filepath + ".enc", "wb") as write_stream:
        write_stream.write(encrypted_content)


You probably want binary mode: "wb" rather than "w".

Consider throwing an exception in place of calling exit(1).

For the filepath[-4:0] == ".enc" enforcement test you might write: filepath.endswith(".enc").

Here is a magic number, corresponding to len(".enc"): filepath[:-4]. Consider changing the enforcement test so it uses a regex, re.compile(r'^(.+)\.enc$'). Then you can use group(1) of the match object when specifying a file path.

The ioError identifier (or io_error according to PEP8) is accurate, but perhaps a bit long -- err would suffice. Better to use the "as" keyword:

except IOError as err: