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.M21·File Extension

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.

Data & Textapplication/mp21

Quick facts

Extension.m21

File typeM21 Data & Text

MIME typeapplication/mp21

CategoryData & Text

Charset—

Compressible—

What is .m21

What is a .m21 file?

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

Use VS Code or Notepad++ to open and edit .m21 files.
Any text editor (Notepad, VS Code) can inspect the raw content as text.

Developer info

When served over HTTP, responses should set the following header:

Content-Type: application/mp21

Common software that reads/writes .m21:

VS CodeNotepad++Sublime Textjq

Relation chain

.m21extension of

format

M21 Data & Textserved as

mime type

application/mp21sent via header

http header

Related extensions

Related MIME types

Common uses

Configuration files
Data interchange between systems
API request and response payloads
Logs and data exports

How to edit .m21 files

Open the file with VS Code, make your changes and save.
Most editors preserve the original formatting and encoding — avoid changing the charset.

How to create .m21 files

Create a new file and save it with the .m21 extension, or export this format from VS Code.

Frequently asked questions

What programs can open .m21 files?

Common options include VS Code, Notepad++, Sublime Text. You can also open many of these files with built-in viewers or a browser.

How do I create a .m21 file?

Create a new file in any text editor or the app that owns the format and save it with the .m21 extension.

What is the relationship between .m21 and the application/mp21 MIME type?

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 2026-09-13

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