7z Archive MIME Type

What does application/x-7z-compressed mean?

The application/x-7z-compressed media type identifies a 7z archive, normally stored with the .7z extension. A 7z file is a container that can bundle files and directories, compress their data, preserve names and timestamps, and optionally encrypt the archive.

MDN lists .7z with application/x-7z-compressed, and the label is widely deployed by browsers, servers, and desktop MIME databases.¹ It is not present in IANA's application media-type registry. The x- spelling is therefore a convention rather than an IANA registration.²

Where did 7z come from?

Igor Pavlov created 7-Zip and 7z. The official source package carries a 7-Zip copyright beginning in 1999 and credits Pavlov, so this page uses 1999 as the format's origin year.³ It is not a standardization date: 7z is an openly documented implementation format, not an IANA-registered media type or a format published by a formal standards body.

How do you identify a 7z archive?

A 7z archive begins with the six-byte signature 37 7A BC AF 27 1C. The next two bytes hold the format version, followed by a CRC and fields that locate the end header.⁴ Checking the signature is more reliable than trusting a filename, but a complete validator must still parse the headers and verify checksums.

The start of a typical archive looks like this:

37 7A BC AF 27 1C 00 04 ...

The 7-Zip documentation describes four logical areas: a 32-byte start header, compressed file data, a compressed metadata block, and an end header.⁴ Metadata can include filenames, sizes, timestamps, compression methods, and CRC values.

Renaming backup.zip to backup.7z does not convert it. ZIP and 7z have different headers and internal structures. Use an archive tool to test the file when its extension and signature disagree.

Compression and encryption in 7z

7z has an open architecture, so an archive records the methods required to decode it. 7-Zip commonly uses LZMA or LZMA2, and the format can also carry Bzip2, PPMd, uncompressed data, executable filters, and other supported methods.⁵

Solid compression lets several files share one compression stream. This can reduce size when the files contain similar data, such as source trees or repeated logs. The tradeoff is less independent access: extracting one item may require decoding earlier data in the solid block, and corruption can make recovery harder. 7-Zip's recovery guidance notes that repairing damaged files in a solid archive is difficult even when good copies of some members are available.⁴

7z also supports AES-256 encryption. 7-Zip derives the encryption key from a password with a SHA-256-based derivation function and repeated hashing.⁵ Encryption does not make a weak password safe, and losing the password generally means losing access to the encrypted data.

How do you open an application/x-7z-compressed file?

Web browsers do not natively browse or extract 7z archives. A response with this media type is normally downloaded for an archive utility to handle.

• On Windows, 7-Zip provides a graphical file manager and command-line tools.
• On macOS, Keka is one option listed by the 7-Zip project. The official 7-Zip download page also provides a macOS console build.
• On Linux, the 7-Zip project publishes console builds for several architectures.⁶
• WinRAR and several other archivers can open 7z files, according to the 7z format page.⁵

With the 7-Zip command-line tool, list or test an archive before extracting it:

7z l package.7z
7z t package.7z
7z x package.7z -ooutput

The x command preserves stored paths. Extract into a new, empty directory so you can inspect the result before mixing it with trusted files.

Support in Poly

Poly recognizes the .7z extension as application/x-7z-compressed and classifies it as an archive. Poly can store, sync, share, version, download, and find the file by name.

Poly deliberately treats the payload as opaque:

• It does not extract the archive or index member filenames and contents.
• It does not generate a thumbnail or offer an in-app archive browser.
• Searches can match the .7z file's name and ordinary Poly properties, but not text packed inside it.
• The Poly agent cannot read files inside the archive.

Download the archive and extract it with a trusted utility when you need its contents. If those extracted files are added to Poly individually, each can enter the processing pipeline appropriate to its own type.

How do you convert a 7z archive?

Archive conversion means extracting the members and creating a new container. It does not convert the files inside.

Choose the destination based on compatibility:

Do not convert an encrypted archive by passing the password to an untrusted online service. Extraction exposes both the password and plaintext contents to that service.

Security and recovery considerations

An archive is untrusted input even when its outer filename looks harmless. Review the member list before extraction, keep your archive software patched, and extract unknown files into an isolated directory. Be cautious of absolute paths, parent-directory components, symbolic links, executables, and archives whose expanded size is far larger than the download.

Encrypted archives can conceal filenames and contents from scanners until the correct password is supplied. After extraction, scan the resulting files and treat executable content with the same caution as a direct download.

7z stores links to metadata near the end of the archive. If the end header or metadata is lost, 7-Zip may be unable to recover filenames and timestamps even when some raw compressed data remains.⁴ Keep another copy of irreplaceable data instead of treating one compressed archive as a backup strategy.

Multi-volume archives often use names such as backup.7z.001, backup.7z.002, and so on. Collect every part in one directory and open the first volume. A missing or shortened part prevents the combined byte stream from reaching the headers and data at their expected offsets.⁴