Indeed, I am unable to download attachments in my Yahoo webmail sessions.
I can only access attachments to emails with “Network Shield” disabled.
I’m thinking someone has been compromised bu downloading a “bad” attachment, and that resulted in attachments being blocked for all users of the server they came through.
I still encountered this issue with a PDF file while downloading it from YMail. I have forwarded the message to a GMail account and the file was downloaded without any problems, not being infected (scanning it after download).