renderRaw (and the attachment-template renderer) sanitised each {token}
with sanitizePart's default 80-char cap, so a single long token such as
{subject} was truncated to 80 — well before the documented 200-char
filename limit, which was therefore unreachable per token. Introduce a
FILENAME_MAX_LEN (200) constant and use it for the per-token cap so the
overall limit governs. Adds tests.