OLE Compound Storage MIME Type

What does application/x-ole-storage identify?

application/x-ole-storage identifies an OLE2 Compound File Binary (CFB) container. Microsoft describes CFB as a general-purpose format that puts a file-system-like hierarchy of storage objects and data streams inside one file.¹ The same container underlies several otherwise unrelated formats, including older Word, Excel, and PowerPoint documents, Outlook messages, and Windows Installer packages.²

The type is best understood as a low-level identification result, not a promise about the payload. The freedesktop.org Shared MIME-info specification uses it as the magic-byte parent of more specific formats. Its example explains that a .doc filename and an OLE signature can combine to identify a Microsoft Word document.³

How can you identify an OLE compound file?

Every CFB file starts with these eight bytes:

D0 CF 11 E0 A1 B1 1A E1

The Microsoft specification requires that signature at offset zero.⁵ It confirms the container, but not the application that created it. You must also inspect the filename and internal directory or streams to distinguish an MSI package from a legacy Office document or another CFB-based format.

This distinction matters for .msi. Microsoft defines .msi as a Windows Installer database extension.⁶ An installation package contains a database, a summary-information stream, and other streams, and it may also carry transforms, source files, or embedded Cabinet archives.⁷ Poly deliberately maps .msi to application/x-ole-storage; the media type itself has no universal extension because the container's extension depends on its subtype.

Why is the date listed as 1993?

Microsoft introduced OLE compound storage into shipping document formats in the early 1990s. Word 6.0 documents from 1993 already used the generic OLE2 Compound Document Format.⁸ The current CFB specification documents two active container versions: version 3 uses 512-byte sectors, while version 4 uses 4,096-byte sectors.²

CFB solved a practical problem for compound documents. Applications could update one stream or embedded object without rewriting every logical component as a separate operating-system file. The result still appeared to users as one document.

How to open or inspect an .msi file

On Windows, Windows Installer opens .msi packages for installation. For inspection, Microsoft's Orca database editor exposes the Windows Installer tables and is supplied with the Windows SDK.⁹

No mainstream browser renders application/x-ole-storage as a document. A browser generally downloads the bytes and lets the operating system choose an application. Renaming the file does not change the container or reveal which kind of CFB payload it contains.

Support in Poly

Poly's MIME table maps .msi to application/x-ole-storage and categorizes it as an archive. That gives the package a known type and lets you store, sync, share, version, and find it by name.

Poly does not parse the Windows Installer database, extract embedded Cabinet streams, generate a package preview, or index the files and actions inside it. The agent therefore cannot answer questions about an MSI's components, properties, or custom actions from the stored package contents.

This is narrower than Poly's document support. A legacy .doc, .xls, or .ppt may use the same CFB signature, but Poly recognizes those extensions as their specific document types and sends them through the document conversion and indexing pipeline.

Is application/x-ole-storage the same as an MSI type?

Not exactly. The relationship is container versus payload:

An MSI and a .doc can share the same first eight bytes while having entirely different internal streams and behavior. Treating all CFB files as installers, or all of them as Word documents, is unsafe and technically incorrect.

Can an OLE storage file be converted?

There is no useful generic conversion for CFB because the container does not define what its streams mean. First identify the subtype, then use a tool that understands it:

• Inspect or extract an .msi with Windows Installer tooling when you need its database or packaged files.
• Open a legacy Office document in a compatible office application, then save it as an appropriate modern document or PDF.
• Preserve the original when its internal metadata, macros, installation logic, or embedded objects matter.

Extracting files from an MSI is not equivalent to installing it. The installation database also describes components, features, conditions, registry changes, and actions that a simple extraction does not reproduce.

Security considerations

An MSI is an executable installation plan, not a passive archive. Windows Installer custom actions may run with elevated privileges in managed or policy-controlled installations.¹⁰ Only install packages from sources you trust, check the publisher's signature where available, and prefer inspection over execution when provenance is uncertain. Microsoft documents digital signatures as a way for Windows Installer to detect altered resources and support publisher-based installation policy.¹¹

The generic CFB signature offers no assurance about safety. It is easy to recognize and shared by many payload types, so validate the specific internal format before choosing a parser or launching an application.