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eFax file format (.efx)

.efx file signature | application/octet-stream

The eFax file format (EFX) is a document format associated with the eFax fax service, originally developed and maintained by eFax, a digital fax provider operated by J2 Global. It is used for sending, receiving, and archiving fax documents in workflows that replace or supplement traditional telephone faxing, including business correspondence and records management. The format is generally considered safe, although like other document files it should be opened only from trusted sources.

Safe

Magic Bytes

Offset 0
DC FE

Sources: Gary Kessler

Extension

.efx

MIME Type

application/octet-stream

Byte Offset

0

Risk Level

Safe

Validation Code

How to validate .efx files in Python

Python 
def is_efx(file_path: str) -> bool:
    """Check if file is a valid EFX by magic bytes."""
    signature = bytes([0xDC, 0xFE])
    with open(file_path, "rb") as f:
        return f.read(2) == signature

How to validate .efx files in Node.js

Node.js 
function isEFX(buffer: Buffer): boolean {
  const signature = Buffer.from([0xDC, 0xFE]);
  return buffer.subarray(0, 2).equals(signature);
}

How to validate .efx files in Go

Go 
func IsEFX(data []byte) bool {
    signature := []byte{0xDC, 0xFE}
    if len(data) < 2 {
        return false
    }
    return bytes.Equal(data[:2], signature)
}

API Endpoint

GET /api/v1/efx
curl https://filesignature.org/api/v1/efx

See the full API documentation for all endpoints and parameters.

Frequently Asked Questions

What is a .efx file?

A .efx file is a eFax file format file. The eFax file format (EFX) is a document format associated with the eFax fax service, originally developed and maintained by eFax, a digital fax provider operated by J2 Global. It is used for sending, receiving, and archiving fax documents in workflows that replace or supplement traditional telephone faxing, including business correspondence and records management. The format is generally considered safe, although like other document files it should be opened only from trusted sources.

What are the magic bytes for .efx files?

The magic bytes for eFax file format files are DC FE at byte offset 0. These bytes uniquely identify the file format regardless of the file extension.

How do I validate a .efx file?

To validate a .efx file, read the first bytes of the file and compare them against the known magic bytes (DC FE) at offset 0. This is more reliable than checking the file extension alone, as extensions can be renamed.

What is the MIME type for .efx files?

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

Is it safe to open .efx files?

eFax file format (.efx) 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.