unsplice-mbox takes an mbox(-ish) file and separates into both a new mbox file without attachments and a directory containing attachments. Instead of the original attachments, emails will have a substitute file indicating the name of the attachment data.
Attachments are decoded (typically from base64) and stored in a content-addressable way.
unsplice-mbox was primarily tested with a single Gmail takeout containing about 100K emails over a span of 20 years. While this contained many different edge cases, it is unlikely that all situations are being handled. Feel free to open an issue or submit a patch if you encounter some parsing errors.
You can get a prebuilt release (Java JRE required) or build it yourself (requires a JDK and Clojure):
make release
ls target/unsplice-mbox# will put attachments into output-mbox.d/
unsplice-mbox input-mbox output-mboxMaking incremental backups of recurring Gmail takeouts is fairly inefficient because the takeout is a single large file and not append-only.