Don't count inline email signature images toward the 100-attachment limit

Customers are hitting Front's 100-attachment error even when they haven't attached many files. Root cause: inline (cid:) images embedded in email signatures (logos, social icons) get carried forward on every reply in a quoted chain and silently stack up, eventually tripping the limit. The only current workarounds are (1) manually stripping quoted history from the reply, or (2) switching every teammate's signature to externally-hosted (linked) images instead of embedded ones. Customers would prefer to not have to do this. Requesting: exclude inline signature images from the attachment count.