What is the MIME file format?
A .mime file stands for Multipurpose Internet Mail Extensions, and is used to encode e-mail messages containing 8-bit data or character sets other than the standard US-ASCII character set. The .mime format allows applications and users to exchange emails with rich content - including formatted text, images, and binary attachments.
A .mime file is plain text structured according to RFC 5322 and the MIME standards (RFC 2045-2049). It begins with mail headers such as From:, Date:, Subject:, and MIME-Version: 1.0, followed by one or more body parts. Binary and non-ASCII content is encoded using Base64 or Quoted-Printable encoding within the body. Multipart messages use a boundary string to separate sections - for example multipart/mixed for messages with attachments, or multipart/alternative for HTML and plain-text variants of the same content.
The format is closely related to the .eml extension used by most email clients; .mime is simply an alternate name for the same RFC 5322 message structure, often produced by command-line tools and some mail servers. If a .mime file cannot be recognized by an e-mail program, it can be opened in any plain text editor - the headers and encoded body will be readable. Email clients such as Mozilla Thunderbird or Microsoft Outlook can render the full message, including any inline images or .mht web archive content.
Security & safety
RISK: MEDIUMThe .mime file itself is plain text and can't run on its own, but the message it holds can carry the usual email threats: malicious attachments (open in a real mail client so it sandboxes/flags them rather than blindly decoding the Base64), phishing links, and remote-image tracking pixels. Treat an unexpected .mime the same as an unexpected email - don't extract attachments or click links unless you trust the source. The headers (Received, SPF/DKIM/DMARC results) can be inspected in a text editor to judge whether the sender is genuine.
Format details
in a nutshell- Generic MIME container / saved web-MIME (MHTML) - Some browsers and tools save MIME-multipart web archives or raw MIME parts with a .mime extension; these are the same RFC 2045 envelope, just wrapping web content instead of an email.
Programs that open MIME files
Windows4 apps
Mozilla Thunderbird
Open-source Free mail client. Rename the file to .eml if needed, then File > Open Saved Message or drag it in to read the message and save attachments. 
Microsoft Outlook
Paid Rename to .eml and double-click to open as a message; save attachments from within. 
Mail (built-in)
Built-in Windows Mail opens MIME/.eml messages; rename to .eml and double-click to view headers, body and attachments.
Any text editor (Notepad, Notepad++)
Free Open the .mime to read the raw headers and text body directly - no client needed for a quick read. 
macOS2 apps
Apple Mail (built-in)
Built-in Rename to .eml and double-click; Mail opens and displays the message and attachments. 
Linux2 apps
Mozilla Thunderbird
Open-source Rename to .eml, then File > Open Saved Message or drag it in. The default choice on Linux. Technical details
deep spec| Format type | Plain-text MIME-encoded email message |
| Governing standard | RFC 2045-2049 (MIME), RFC 5322 (Internet Message Format) |
| MIME content type | message/rfc822 |
| File signature / magic bytes | None - plain text with no fixed byte sequence; typically begins with a header line such as From:, Date: or MIME-Version: 1.0 |
| Header format | RFC 5322 field-name/value pairs, one per line, separated from the body by a blank line |
| Required MIME header | MIME-Version: 1.0 must appear in every MIME-compliant message |
| Body encoding | Base64 or Quoted-Printable for binary and non-ASCII content; 7-bit ASCII parts may remain unencoded |
| Multipart support | Yes - multipart/mixed, multipart/alternative, multipart/related, multipart/signed, and others |
| Line terminator | CRLF (\r\n) required per RFC 5322 for transport; local storage may use LF only |
| Maximum line length | 998 characters hard limit; 78 characters recommended (RFC 5322 §2.1.1) |
| Character set support | Any charset declared via the charset parameter in Content-Type (e.g., UTF-8, ISO-8859-1, Windows-1252) |
| Attachment encoding | Binary files are Base64-encoded MIME parts declared with Content-Disposition: attachment and a filename parameter |
| Boundary delimiter | A unique string declared in the Content-Type header that separates parts in multipart messages |
| Interoperability | Handled by all RFC-compliant mail transfer agents (MTAs), email clients, and mail processing libraries |
| Released | MIME RFC 2045+ (1996); built on RFC 822 (1982) |
| Open standard | Yes · royalty-free |
| Specification | www.rfc-editor.org |
MIME conversions
Community Q&A
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