$decode() halts when it hits a character that isn't part of the base64 set. This contradicts the base64 section of the MIME RFC (section 6.8) which states that they should be ignored. This causes a lot of interoperability issues since another part of the base64 section directs that base64-encoded output should be longer than 76 characters-per-line, meaning most base64 encoded output contains a whole load of CRLFs.

$remove

Using $remove() requires the encoded content to be in a text variable, which, if you're decoding a file means switching from simply using bread to using filter -k or $read() and a while loop. That not only causes major slowdown but more importantly still doesn't solve the problem of mIRC not correctly supporting the base64 encoding.