ripMIME may prematurely terminate decoding Base64 encoded
messages when it encounters multiple blank lines or other
non-standard Base64 constructs. Virus scanning and content
filtering tools that use ripMIME may therefore be
bypassed.
The ripMIME CHANGELOG file says:
There's viruses going around exploiting the ability to
hide the majority of their data in an attachment by using
blank lines and other tricks to make scanning systems
prematurely terminate their base64 decoding.