What is MIME type "application/x-compressed-iso"?

A MIME type is a string that tells browsers and other tools how to handle a particular kind of file.

The MIME type application/x-compressed-iso labels files that hold compressed disk images. This means an original ISO image is stored in a smaller, compressed format while keeping its structure intact.

It is commonly used when backing up or emulating optical media. Software that reads these images must support the compression. The technique helps save space and speeds up loading times in many cases.

Main use: Efficient backup of optical disc images.
Emulation: Supports systems like game console emulators that load disc images.
Ensures the original file system structure is preserved in a compressed format.
Reduces file size without sacrificing the ability to mount or run the image.

Files marked with this MIME type usually carry extensions such as CSO, CISO, or WBI. These links provide more details on each specific format.

Associated file extensions

.cso, .ciso, .wbi

Usage Examples

HTTP Header

When serving content with this MIME type, set the Content-Type header:


    Content-Type: application/x-compressed-iso

HTML

In HTML, you can specify the MIME type in various elements:


    <a href="file.dat" type="application/x-compressed-iso">Download file</a>

Server-side (Node.js)

Setting the Content-Type header in Node.js:


    const http = require('http');    
    
    http.createServer((req, res) => {    
      res.setHeader('Content-Type', 'application/x-compressed-iso');    
      res.end('Content here');    
    }).listen(3000);

Associated file extensions

.cso, .ciso, .wbi

FAQs

What is the application/x-compressed-iso MIME type?

This MIME type identifies compressed disk image files, most commonly associated with the CSO format used in emulation. It tells the system that the file is an optical disc image (like an ISO) that has been compressed using a specific algorithm to save storage space while retaining the file structure.

How do I configure Apache to serve .cso files correctly?

Since this is a non-standard MIME type, you must add it to your .htaccess file or httpd.conf. Add the line AddType application/x-compressed-iso .cso .ciso .wbi to ensure the server sends the correct header, prompting a download rather than displaying garbage text.

What is the difference between CSO and standard ISO files?

A CSO file is simply a compressed version of a standard ISO file. While CSO files take up less disk space—making them ideal for portable devices like the PSP—they require processing power to decompress on the fly, which can occasionally cause minor lag in emulation compared to uncompressed ISOs.

Can web browsers open application/x-compressed-iso files?

No, web browsers like Chrome or Firefox cannot natively open or mount compressed disk images. When a browser encounters this MIME type, it will automatically start downloading the file so it can be used with local software like the PPSSPP emulator or a virtual drive tool.

Which software opens files with the .cso extension?

Files with the .cso extension are primarily opened by game console emulators such as PPSSPP (PlayStation Portable Simulator). They can also be manipulated or converted back to standard ISO format using utilities like UMDGen or CISO Converter.

Why use application/x-compressed-iso instead of application/octet-stream?

Using the specific application/x-compressed-iso type provides more semantic meaning than the generic application/octet-stream. It helps client-side applications identify that the binary stream is specifically a compressed disk image, potentially allowing file associations with emulation software to trigger automatically.

Is application/x-compressed-iso an official IANA standard?

No, the x- prefix indicates that it is a non-standard or experimental subtype. Because it is not part of the official IANA registry, web servers (like Nginx or IIS) usually do not recognize it by default and require manual configuration to associate it with extensions like .cso or .wbi.

General FAQ

What is a MIME type?

A MIME (Multipurpose Internet Mail Extensions) type is a standard that indicates the nature and format of a document, file, or assortment of bytes. MIME types are defined and standardized in IETF's RFC 6838.

MIME types are important because they help browsers and servers understand how to process a file. When a browser receives a file from a server, it uses the MIME type to determine how to display or handle the content, whether it's an image to display, a PDF to open in a viewer, or a video to play.

MIME types consist of a type and a subtype, separated by a slash (e.g., text/html, image/jpeg, application/pdf). Some MIME types also include optional parameters.

How do I find the MIME type for a file?

You can check the file extension or use a file identification tool such as file --mime-type on the command line. Many programming languages also provide libraries to detect MIME types.

Why are multiple MIME types listed for one extension?

Different applications and historical conventions may use alternative MIME identifiers for the same kind of file. Showing them all helps ensure compatibility across systems.