When messages are marked as spam, Xeams captures the signatures of the attached images and marks them as well. If the same image is received again, it assigns a score. If you are seeing a good message containing "Message contains a black-listed attachment", that means someone has incorrectly marked a good message containing an image.

This information is stored in $INSTALL_DIR\config\imageFP.dat file. If you remove this file, Xeams will lose this fingerprint information and it won't happen again.