That's because we don't actually support AZW3 (i.e., KF8), only AZW (i.e., M7, a.k.a., legacy Mobi), yet they apparently share a mimetype or two ("application/vnd.amazon.mobi8-ebook" & "application/x-mobi8-ebook").
And "support" is kind of a strong word here (i.e., don't do this, just read ePubs).
If there are more precise mimetypes available (and they're actually used properly by OPDS servers), I'm all ears

.