Find MIME Type Associations in Windows Registry

This article explains the exact Windows Registry locations where system-level MIME (Multipurpose Internet Mail Extensions) content type associations are defined and stored. Understanding these registry paths allows administrators and developers to inspect, modify, or troubleshoot how the operating system and installed applications map specific media types to file extensions.

Primary System-Level Registry Path

System-level MIME content type associations are located in the Registry under the following path:

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\MIME\Database\Content Type

Because HKEY_CLASSES_ROOT represents a merged view of machine-wide and user-specific configurations, you can also view these associations at:

HKEY_CLASSES_ROOT\MIME\Database\Content Type

Registry Structure and Values

Under the Content Type key, each supported MIME type exists as an individual subkey formatted as type/subtype (for example, application/pdf, image/png, or text/html).

Within each MIME type subkey, you will find specific registry values:

• Extension (REG_SZ): Specifies the primary file extension associated with the MIME type (e.g., .pdf, .png, .html).
• CLSID (REG_SZ): (Optional) Contains the Class Identifier of the COM component registered to handle the data stream for that MIME type.
• Encoding (REG_BINARY / REG_DWORD): (Optional) Defines the character encoding or binary handling flags used by Windows networking components.

Reverse Association: File Extension to MIME Type

Windows also defines the reverse association—mapping a specific file extension back to a MIME content type. This is stored within the individual file extension subkeys at the following path:

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\<.extension>

For example, checking HKEY_LOCAL_MACHINE\SOFTWARE\Classes\.json will reveal a string value named Content Type set to application/json.

Accessing and Editing the Keys

1. Press Win + R, type regedit, and press Enter to open the Registry Editor.
2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Classes\MIME\Database\Content Type.
3. Locate or create a subkey named after the desired MIME format to review or update its Extension string value.