.msi File Extension

What kind of file is a .msi?

An .msi file is a Windows Installer package. It contains a relational installation database, a summary-information stream, and other streams that tell Windows Installer how to install, modify, repair, or remove a product. The package can carry source files and transforms, while larger payloads may live in embedded or external CAB archives.¹

Poly associates the suffix with the de facto application/x-ole-storage MIME type. MSI has no dedicated IANA-registered media type.² Its underlying Compound File Binary container starts with D0 CF 11 E0 A1 B1 1A E1, but legacy Office documents and other compound files share that signature.³⁴ Confirm the extension and internal database structure before treating a file as an installer.

How do I open a .msi on my computer?

On Windows, double-click a trusted MSI to open it with Windows Installer. The command msiexec.exe /i "C:\path\package.msi" starts a normal installation; msiexec also supports repair, uninstall, logging, and administrative-install options.⁵

To inspect a package without installing it, use Microsoft's Orca database-table editor from the Windows SDK or a package-aware extraction utility.⁶ On Windows, 7-Zip can expose some packaged files, but it lists MSI as an unpack-only format.⁷ On macOS and Linux, third-party package inspection tools can reveal package data, but they do not perform a Windows installation. A virtual machine is the safer choice when you need to test actual installer behavior.

Poly support for .msi

Poly stores, syncs, shares, versions, and downloads MSI packages. They remain searchable by filename and ordinary file properties.

Poly does not unpack embedded cabinets, preview the installer, index its database tables or payload, or expose those contents to the Poly agent. Download the package and inspect it with a trusted Windows Installer tool when you need its components, properties, conditions, or actions.

History of .msi

MSI belongs to Microsoft's database-driven Windows Installer system. Instead of being just a self-extracting program, a package describes products through tables for files, components, features, directories, registry changes, services, and ordered actions.⁹

The platform evolved alongside Windows. Windows Installer 2.0 shipped with Windows XP, 4.0 with Windows Vista, and 5.0 with Windows 7 and later releases.¹⁰ Microsoft also assigned related suffixes such as .msp for patches, .mst for transforms, and .msm for merge modules. These are related Windows Installer artifacts, not interchangeable names for an MSI.³

Working with .msi and alternatives

Choose the artifact that matches the target:

Extracting an MSI can help with authorized inspection, but it is not equivalent to installing it. The database may define feature selection, registry changes, services, shortcuts, conditions, and custom actions that extraction skips. Keep the original signed package when provenance or reliable repair and uninstall behavior matters.