What is MIME type "multipart/appledouble"?

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

multipart/appledouble is a MIME type that encapsulates a file split into separate parts. One part holds the file’s primary data while the other contains Mac-specific metadata such as resource forks and extended attributes.
This separation ensures that extra Mac information is not lost when the file is shared on systems that do not support such details.

Functionality involves packaging a file into dual components. This is important when a file coming from a Mac system needs to be transferred to non-Mac environments.
It prevents issues like data corruption or loss during email transmissions and cross-platform transfers.

Preservation: Keeps metadata like icons and file comments intact.
Compatibility: Enables Mac files to be used on Windows or Linux without losing essential file details.
File Transport: Often applied to email attachments and archives where the full Mac file structure must be maintained.

This MIME type is associated with file types such as ADF, RSRC, and AD.

Associated file extensions

.adf, .rsrc, .ad

Usage Examples

HTTP Header

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


    Content-Type: multipart/appledouble

HTML

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


    <a href="file.dat" type="multipart/appledouble">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', 'multipart/appledouble');    
      res.end('Content here');    
    }).listen(3000);

Associated file extensions

.adf, .rsrc, .ad

FAQs

What is the primary purpose of the multipart/appledouble MIME type?

This MIME type is used to safely transport Macintosh files across networks or systems that do not support Mac-specific file attributes. It splits the file into two parts: a Data Fork (the actual content) and a Resource Fork (metadata like icons and window positions), ensuring no information is lost during transfer.

How do I configure Apache to recognize AppleDouble files?

You can map the associated file extensions to the MIME type in your .htaccess file or main configuration. Add the line AddType multipart/appledouble .adf .rsrc to ensure the server identifies these files correctly when served.

Why do I see files starting with '._' associated with this MIME type?

When macOS writes files to non-HFS+ file systems (like FAT32 USB drives or network shares), it creates hidden files starting with ._ using the AppleDouble format. These files contain the resource fork data that the destination file system cannot store natively.

Can web browsers display multipart/appledouble content directly?

No, standard web browsers like Chrome or Firefox do not render multipart/appledouble content natively. When a browser encounters this MIME type, it will typically prompt the user to download the file rather than displaying it within the window.

Is it safe to delete AppleDouble files on a Windows server?

Generally, yes, if you only need the actual file content (data fork). However, deleting the AppleDouble part (often the .rsrc or ._ file) removes Mac-specific metadata, which may result in missing icons or type information if the file is moved back to a Macintosh system.

What is the difference between AppleDouble and AppleSingle?

While AppleDouble splits the file into two separate components (data and header/resource), AppleSingle combines both the data fork and the resource fork into a single file structure. AppleDouble is often preferred for cross-platform compatibility because non-Mac systems can still easily access the data part.

How does Nginx handle multipart/appledouble files?

Nginx requires the MIME type to be defined in the mime.types file to serve it with the correct header. You should ensure the entry multipart/appledouble adf rsrc; exists in your configuration to prevent the server from defaulting to application/octet-stream.

Are there security risks associated with multipart/appledouble?

The primary risk involves metadata leakage. Since the resource fork can contain information about the file's origin, author, or internal paths, you should inspect these files before sharing them publicly to ensure no sensitive internal data is inadvertently exposed.

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.