I confirmed the attachment filter does not have a .xlsx entry added. I disabled all entries, restarted the transport service, and resent the source file. It was still stripped.
I wasn't able to locate a reason in the logs here, or anywhere else:
Edge Server: C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\Edge\ProtocolLog\SmtpSend
Disabling the attachment filter agent allows file to pass, so I was able to get a copy of the affected source file.
I am able to successfully send personal .xlsx files, so to me, it's how this particular .xlsx is being generated.
This FIXES my issue, but I'm on EX2013 CU21
https://support.microsoft.com/en-gb/help/2855809/email-message-attachments-are-removed-incorrectly-on-an-exchange-serve
I extracted a normal .xlsx and compared the [Content_Types].xml
New .xlsx
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">
<Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/>
<Default Extension="xml" ContentType="application/xml"/>
<Override PartName="/xl/workbook.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"/>
<Override PartName="/xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"/>
<Override PartName="/xl/theme/theme1.xml" ContentType="application/vnd.openxmlformats-officedocument.theme+xml"/>
<Override PartName="/xl/styles.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml"/>
<Override PartName="/docProps/core.xml" ContentType="application/vnd.openxmlformats-package.core-properties+xml"/>
<Override PartName="/docProps/app.xml" ContentType="application/vnd.openxmlformats-officedocument.extended-properties+xml"/>
</Types>
Stripped .xlsx
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">
<Default ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.printerSettings" Extension="bin"/>
<Default ContentType="image/png" Extension="png"/>
<Default ContentType="application/vnd.openxmlformats-package.relationships+xml" Extension="rels"/>
<Default ContentType="application/xml" Extension="xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml" PartName="/xl/workbook.xml" />
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml" PartName="/xl/worksheets/sheet1.xml"
/>
<Override ContentType="application/vnd.openxmlformats-officedocument.theme+xml" PartName="/xl/theme/theme1.xml" />
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml" PartName="/xl/styles.xml" />
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sharedStrings+xml" PartName="/xl/sharedStrings.xml"
/>
<Override ContentType="application/vnd.openxmlformats-officedocument.drawing+xml" PartName="/xl/drawings/drawing1.xml" />
<Override ContentType="application/vnd.openxmlformats-package.core-properties+xml" PartName="/docProps/core.xml" />
<Override ContentType="application/vnd.openxmlformats-officedocument.extended-properties+xml" PartName="/docProps/app.xml" />
<Override ContentType="application/vnd.openxmlformats-officedocument.custom-properties+xml" PartName="/docProps/custom.xml" />
</Types>
What gives?