The problem you are having is because the lines are too long. Mail messages are limited to lines of 998 characters (1000 characters with the CRLF).

The Content-Type is irrelevant to the encoding. The type is for the client to determine how to present the attachment. It sounded you are sending a text file; text/plain is likely the right type to use.

If you can't shorten the lines in the file, then you need to use an Content-Transfer-Encoding. If you want to preserve the file, then base64 is the right encoding to use. If you just care about protecting high-bit characters and line lengths but don't care about control characters, then quoted-printable may work. The advantage of quoted-printable is that the attachment is still readable, just with non-ASCII values and line endings encoded with the equals signs you sometimes see.