Base64 / MIME file format
.b64 · .eml · binary-to-text encoding · introduced 1992 by IETF (RFC 1341)
What is a Base64 / MIME file?
Base64 is the binary-to-text encoding standardized for MIME email attachments in 1992. Although not strictly an archive format, it is how every binary file you have ever attached to an email actually crossed the wire. Base64 is also widely used in JSON, JWT, and embedded image data URIs.
Most readers reach this page after double-clicking a .b64 file and finding their operating system unsure what to do with it. The good news: Base64 / MIME is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.
How to extract a Base64 / MIME file
Opening a .b64 file is straightforward on every modern operating system. The exact steps depend on which platform you are on:
On Windows
- Locate the
.b64file in File Explorer. - Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
- If prompted, pick a destination folder and click Extract.
- The extracted files appear in a folder next to the original archive.
On macOS
- Find the
.b64file in Finder. - Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
- Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
- The extracted folder appears next to the archive.
On Linux
- Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).
- On the command line, run one of:
7z x archive.b64 # works for almost any format
- The files appear in the current directory.
How to create a Base64 / MIME file
To pack files into a .b64 archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write Base64 / MIME.
On the command line, the canonical create command is:
# Use 7-Zip, PeaZip, or the format's reference tool to create .b64 files.
If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.
Strengths
- Universal
- Standardized
- Trivial to encode/decode
Weaknesses and limitations
- 33% size overhead
- Not compression
Typical use cases
- MIME email attachments
- Data URIs
- JWT tokens
- Inline image embedding in HTML/CSS
Technical details
Base64 / MIME uses the Base64 (3 bytes -> 4 chars) algorithm and is identified on disk by the magic byte sequence (none). The standard MIME type is text/plain. The format is an open specification and is free for any use.
Frequently asked questions
Is Base64 / MIME safe to open?
Archive files are containers — they are only as safe as what they contain. A .b64 from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.
What is the maximum file size?
Modern implementations support archives in the multi-terabyte range. The practical limit is your filesystem (FAT32 caps at 4 GB; ext4, NTFS, APFS, and exFAT all comfortably handle multi-TB files).
Can I password-protect a Base64 / MIME?
Some archivers can wrap Base64 / MIME files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.
How does Base64 / MIME compare to other formats?
See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.