You can't "block" attachments. On the wire, they are just a series of bytes. They can not resolved to be attachments until the message has been received and parsed.

You can create filters which might be able to detach or delete these attachments, but in practice I have found it impossible to use filters on the meta-data segments of a message where content descriptors are found. This means it may not be possible to detect the type of attachment.

Are you aware that jpegs can carry unwanted payload? Of the types you list, I suspect pdf is the least likely to be able to carry any risky content.