Signed messages downloaded from Hyperkitty are missing the message body.
For example see the thread at https://lists.mailman3.org/archives/list/mailman-users@mailman3.org/thread/2WHXO43OV2SYJFRTYNBZONQHKHJ6KVAJ/.
When downloaded, the first two (signed) messages in the thread only contain an application/pgp-signature part. The text/plain body is missing from the download.
I didn't check other attachment types.