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Office files like .docx, .xlsm, ... are Office Open XML (OOXML) files: a ZIP container containing XML files and possibly other file types.

OOXML files follow the Open Packaging Conventions (OPC) format.

OPC files contain a /[Content_Types].xml file (describing the MIME format of all parts of the OPC container) and a _rels/.rels file (documenting the relationships inside the OPC container).

Like this .xlsm file:

In my experience with OOXML files, /[Content_Types].xml is the first ZIP record, and _rels/.rels is the second ZIP record.

When an OOXML file has been modified with a ZIP utility, it's often the case that that order is no longer respected: files /[Content_Types].xml  and _rels/.rels  are no longer first and second (this has no impact on the parsing of these altered files by Office applications).

AFAIK, the OPC standard does not require these 2 files to be the first in the ZIP container.

Please post a comment if you know of OPC examples (there are other file formats than OOXML that are based on OPC) created by applications that do not put these 2 files first inside the ZIP container.

Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com DidierStevensLabs.com