Features

Automatically attaches files from the original email when replying.
Configurable behavior: attachments can be
- added automatically, or
- added only after confirmation (a small, accessible dialog). In Options you can enable the confirmation and choose the default answer (Yes/No).
Blacklist of filenames (glob patterns) prevents specific files from being attached automatically. Examples: *intern*, *secret*, *passwor*. Matching is case-insensitive and checks the filename only; provide one pattern per line in Options.
Blacklist warning (optional, enabled by default): when files are excluded by your blacklist, a small modal lists the file and the matching pattern(s). Dark-mode friendly and keyboard accessible (Enter/Esc to close).
Works with Reply and Reply all. Forward is not modified by this add-on.
Adds originals even if you already attached something yourself; avoids duplicates by filename.
Per-tab duplicate guard prevents double-adding in the same compose tab.
Skips S/MIME certificates by default to avoid unnecessary attachments.
Embedded images are left to Thunderbird: they stay in the reply body and are not copied as file attachments. An image that only carries a Content-ID without being referenced is treated as a normal attachment and copied.

How It Works

On reply, the add-on lists original attachments.
Filters out S/MIME signatures from file attachments; inline images are restored in the body (unless disabled).
Optionally asks for confirmation (keyboard-friendly).
Adds eligible files to your compose, avoiding duplicates by filename.
See "Why attachments might not be added" in Usage for edge cases.

Privacy note: All processing happens locally in Thunderbird. The add-on makes no background network requests.