AOL HTML mail file (.dci)

.dci file signature | application/octet-stream

AOL HTML mail file (DCI) is a legacy email message format created and used by AOL for storing HTML-based mail content. It was used in AOL client software to display formatted messages, embedded links, and other browser-rendered content within the mail application. The format is generally safe to open, but as a legacy HTML-based mail type, it may carry active content or outdated rendering behavior in older readers.

Safe

Magic Bytes

Offset 0


              3C 21 64 6F 63 74 79 70

Sources: Gary Kessler

Extension

.dci

MIME Type

application/octet-stream

Byte Offset

Risk Level

Safe

Validation Code

How to validate .dci files in Python

Python

def is_dci(file_path: str) -> bool:
    """Check if file is a valid DCI by magic bytes."""
    signature = bytes([0x3C, 0x21, 0x64, 0x6F, 0x63, 0x74, 0x79, 0x70])
    with open(file_path, "rb") as f:
        return f.read(8) == signature

How to validate .dci files in Node.js

Node.js

function isDCI(buffer: Buffer): boolean {
  const signature = Buffer.from([0x3C, 0x21, 0x64, 0x6F, 0x63, 0x74, 0x79, 0x70]);
  return buffer.subarray(0, 8).equals(signature);
}

How to validate .dci files in Go

func IsDCI(data []byte) bool {
    signature := []byte{0x3C, 0x21, 0x64, 0x6F, 0x63, 0x74, 0x79, 0x70}
    if len(data) < 8 {
        return false
    }
    return bytes.Equal(data[:8], signature)
}

API Endpoint

GET /api/v1/dci


              curl https://filesignature.org/api/v1/dci

See the full API documentation for all endpoints and parameters.

Frequently Asked Questions

What is a .dci file?

A .dci file is a AOL HTML mail file file. AOL HTML mail file (DCI) is a legacy email message format created and used by AOL for storing HTML-based mail content. It was used in AOL client software to display formatted messages, embedded links, and other browser-rendered content within the mail application. The format is generally safe to open, but as a legacy HTML-based mail type, it may carry active content or outdated rendering behavior in older readers.

What are the magic bytes for .dci files?

The magic bytes for AOL HTML mail file files are 3C 21 64 6F 63 74 79 70 at byte offset 0. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .dci file?

To validate a .dci file, read the first bytes of the file and compare them against the known magic bytes (3C 21 64 6F 63 74 79 70) at offset 0. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .dci files?

There is no officially registered MIME type for .dci files. Systems typically use application/octet-stream as a generic fallback when handling this format.

Is it safe to open .dci files?

AOL HTML mail file (.dci) files are generally safe to open. They are classified as low risk because they primarily contain data rather than executable code. However, always ensure files come from a trusted source.