What's Happened to Gmail spam filtering? Why is gmail spam filter not working?

Over the last week or two Gmail spam filtering seems to have taken a turn for the worse. Gmail is normally very good at preventing spam getting to your inbox but recently many messages that appear to be OBVIOUS spam have been deluging the Gmail inbox. So why is the Gmail spam filter not working?

Repeated messages offering Russian wives, gambling accounts and medicines with clear text in the email are being received direct into inboxes not spam.

Messages received recently include the text below

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Dear Gentleman!

You've got 46 more fresh messages from girls at our site today:
[ Read here ]
For any questions and offers, please, contact me!
Administrator Galina

Gmail Spam Explosion

The normally very successful Gmail spam filter seems to have been overloaded with a new variety of spam in the last week or so. Initially it was spam emails with just the words "hello how are you" and no other information that were getting into our inbox without being filtered. It was unclear what the purpose of these messages was as there was no advertising, no links and no attachments and the sender address was also spoofed.

The latest messages that are defeating the Gmail spam filter are a series with contents as below

Please find attached below a copy of the document you requested.
Please find my CV and cover letter attached.
Greetings from Amazon.com

All have a file attached of the format below which may have some payload attached if run on your PC but on iPhone just shows as text.

27915doc.html

The messages have all been reported to Gmail to improve their filtering but it is unclear what the purpose is when replies cannot be harvested for active email accounts and the email contains no payload.