.bin files being embeeded into .xlsx attachments in emails

There are a number of extensions our mail filters block for inbound emails;, .exe, .bat...One of those extensions by default (in the program we us:  Symantec Mail Security for SMTP) is a .bin file.  

On two occasions, someone has sent us a .xlsx and a .bin file has been removed from the email and the spradsheet will not open.  I know I could just allow .bin files and it would work but what do you think is happening here?  Why is there a .bin file in the email and what does that have to do with the .xlsx spreadsheet?

Thanks in advance!
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Rory ArchibaldCommented:
bin files are created for macros and OLE objects embedded in the files. If it's definitely an .xlsx and not an .xlsm, then the bin file presumable relates to an embedded object, not a VBA project.
vianceadminAuthor Commented:
Yes, it's definitely a .xlsx file.  So the .bin file is needed to open the spreadsheet?  The sender doesn't know that he's sending the .bin file...He's just sending the spreadsheet...
Rory ArchibaldCommented:
Do you know the name of the .bin file that is being removed? The .bin file will be part of the file - it's created automatically when he saves it as an .xlsx (which is effectively a zip file full of other files) - is he embedding any other files or objects in the workbook?
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vianceadminAuthor Commented:

I believe that this is used in Office 2007 documents to compress the file (read that somewhere).  
Rory ArchibaldCommented:
Oh yes, I forgot about printer settings! Yes, they are always stored as bin files. There's quite a good list of things that can be included as bin files here:
PS You could probably tell your mail security to ignore xlsx files?

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vianceadminAuthor Commented:
The mail security is triggering on the .bin file not the .xlsx file.  Also found this:

Email Virus Scanner Quarantines Office 2007 Documents
When a user receives a Office 2007 documents saved in the Office OpenXML format (e.g. .docx, .xlsx, pptx) as an email attachment, occasional files may be quarantined or otherwise flagged as infected by an email antivirus program. This is caused by the virus scanner flagging the file because it contains a binary file (.bin).

The Office OpenXML format files are actually zip files which contain multiple files within them. Antivirus software may recognize them as such, unzip them, and analyze the contents individually. Among the files which an Office OpenXML document can contain are files such as oleObject1.bin (containing embedded OLE objects) or PrinterSettings1.bin (containing printer settings). These .bin files may cause the attachment to trigger a policy in the virus scanner which blocks executables or filters common executable extensions (.exe, .bin, .com, .bat, .pif, etc.).

Potential workarounds include turning off quarantining of executable attachments, allowing attachments with the .bin extension, or not scanning zips, but both of these are potentially too dangerous to seriously consider. Until the Office OpenXML format becomes more prevalent and/or the antivirus vendors find an intelligent solution to this issue, it may be easier to individually release these files from quarantine.

I don't really want to not scan .bin attachments but there doesn't seem to be much of a workaround...

Thanks for your help!
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