Banned messages with PDF attachments

[Nikolaos Milas]

Hi,

I have some messages with pdf attachments banned. I can see:

X-Amavis-Alert: BANNED, message contains 
application/x-msdownload,.pdf,2nd Circular_IEC2013.pdf

The attachment has:

Content-Type: application/x-msdownload;  name="2nd Circular_IEC2013.pdf"

Why such messages get banned (although I think they shouldn't)?

What is their difference from other messages with PDF attachments like 
the following, that reach us without issues:

Content-Type: application/pdf; 
name="=?UTF-8?B?zqbOlc6aX86RXzE2My0yMDEwLnBkZg==?="

When a PDF file attachment is characterized as 
"application/x-msdownload" (and it gets banned) and when 
"application/pdf" (and it doesn't)?

Please advise.

Thanks,
Nick