How Are Email Attachments Encoded?

How Are Email Attachments Encoded?

Email attachments are encoded using MIME (Multipurpose Internet Mail Extensions) encoding, typically Base64 encoding. Attachments are converted to text format using Base64 encoding so they can be transmitted as part of email messages.

The encoding process converts binary file data into ASCII text characters, allowing attachments to be included in email body or as separate MIME parts. Encoding information is included in email headers and MIME structure, allowing email clients to decode and extract attachments properly.

Extract and decode attachments from emails using our extract email attachments tool to analyze attachment structure and content.

MIME Encoding

What Is MIME?

MIME (Multipurpose Internet Mail Extensions) is a standard that extends email format to support attachments, non-ASCII characters, and multiple content types.

MIME Structure

• Content-Type headers specify attachment type
• Content-Disposition headers indicate attachment handling
• Content-Transfer-Encoding specifies encoding method
• MIME boundaries separate email parts

MIME Parts

Attachments are included as separate MIME parts within multipart email messages, each with its own headers and encoded content.

Content Types

MIME content types (e.g., application/pdf, image/jpeg) identify attachment file types and how they should be handled.

Base64 Encoding

What Is Base64?

Base64 is an encoding scheme that converts binary data into ASCII text characters, making it safe for transmission in email messages.

Encoding Process

• Binary file data is read
• Data is converted to Base64 text format
• Encoded text is included in email body
• Email clients decode Base64 back to binary

Why Base64?

Base64 encoding ensures attachments can be transmitted through email systems that only support text, while preserving file integrity.

Size Increase

Base64 encoding increases file size by approximately 33%, as binary data is converted to text representation.

Attachment Structure

Attachments in emails follow a specific structure:

MIME Headers

• Content-Type: File type and MIME type
• Content-Disposition: Attachment or inline
• Content-Transfer-Encoding: Encoding method (usually base64)
• Content-ID: Unique identifier for inline attachments

Encoded Content

Attachment file data encoded as Base64 text, typically wrapped at 76 characters per line for email compatibility.

MIME Boundaries

Boundaries separate attachment parts from email body and other parts in multipart messages.

Extracting Attachments

Extracting attachments requires decoding Base64 encoding and parsing MIME structure:

Extraction Process

• Parse email MIME structure
• Identify attachment parts
• Extract Base64 encoded content
• Decode Base64 to binary data
• Save as original file format

Extraction Tools

Use our extract email attachments tool to automatically extract and decode attachments from email files (.eml, .msg) or email source code.

Manual Extraction

Attachments can be manually extracted by parsing MIME structure, finding Base64 content, and decoding it using Base64 decoders.