M21 Data & Text — A .m21 file is a M21 Data & Text. It is a text-based data format widely used for configuration files, data interchange and API payloads.
Quick facts
What is .m21
A .m21 file is a M21 Data & Text. It is a text-based data format widely used for configuration files, data interchange and API payloads.
In the mime-db registry it maps to application/mp21. When serving .m21 over HTTP, set the Content-Type header as shown in Developer info below.
Related entities: it maps to application/mp21 — and is delivered over HTTP via the Content-Type header.
How to open .m21 files
Developer info
When served over HTTP, responses should set the following header:
Content-Type: application/mp21
Common software that reads/writes .m21:
Relation chain
Related extensions
Related MIME types
Common uses
How to edit .m21 files
How to create .m21 files
Frequently asked questions
Common options include VS Code, Notepad++, Sublime Text. You can also open many of these files with built-in viewers or a browser.
Create a new file in any text editor or the app that owns the format and save it with the .m21 extension.
application/mp21 is the media type identifier used for .m21 files during network transfer (for example in the HTTP Content-Type header).
Sources & standards
Updated 2026-08-18
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