Don’t get me wrong, I like the feature of Thunderbird (and other email clients) that looks for keywords indicating you intended to attach a document, and warns you that you haven’t.
But I think it needs tweaking.
The attachment keyword “pdf” is clearly part of a quoted URL in this email. In these cases, I think there’s no need to give me the warning.
Yeah, if I were energetic enough, I’d report the bug and/or fix it myself.
(Oh, whatdayaknow, I just patched to the latest version of TBird, and it looks like someone fixed it. Still, a little more quality control wouldn’t hurt to ensure this type of bug wasn’t released into a “stable” version in the first place.)