Lzip MIME Type

What does application/x-lzip mean?

The application/x-lzip media type identifies data compressed in the Lzip format, usually in a file ending in .lz. Lzip is lossless, so successful decompression reproduces the original bytes. It uses a constrained form of LZMA compression inside a format designed for interoperability, integrity checking, and recoverability.¹

application/x-lzip is a deployed but unregistered media type. The shared-mime-info database used by many Linux desktops associates it with the .lz extension and the LZIP file signature, while IANA's current application registry contains neither x-lzip nor lzip.²³

Where did Lzip come from?

Antonio Diaz Diaz designed Lzip in 2008 as a Unix-style alternative to Gzip and Bzip2.⁴ Its command-line interface follows familiar compressor conventions, but its file format adds a fixed-size header and a trailer containing three integrity values.

The format remains independently documented and implemented. The official manual includes a small reference decompressor, and the Lzip project publishes implementations for single-threaded, parallel, library, and recovery-oriented use cases.¹⁵

There is an active Internet-Draft that proposes the registered name application/lzip and describes application/x-lzip as its deprecated alias. That proposal has not yet become an RFC or an IANA registration, so application/x-lzip remains the spelling used by Poly and shared-mime-info today.³⁶

How do you identify a Lzip file?

Every Lzip member starts with the four ASCII bytes LZIP, or 4C 5A 49 50 in hexadecimal. The next byte is the format version, currently 01, and the sixth byte encodes the dictionary size.¹

4C 5A 49 50 01 xx ...
 L  Z  I  P

A Lzip file can contain one or more independent members concatenated together. Each member ends with a 20-byte trailer containing:

• A CRC32 of the uncompressed data.
• The uncompressed data size.
• The complete member size.

Together with the LZMA end marker, those fields let a decoder cross-check the stream in three ways. The member-size field also allows tools to walk a multi-member file backward and helps recovery software locate intact members.¹

The signature is a useful identification check, but it is not proof that the entire stream is valid. Test the complete file with Lzip before relying on it:

lzip -tv download.lz

How do you open an application/x-lzip file?

Web browsers do not provide a native Lzip viewer or decompression API. A server response using application/x-lzip is normally downloaded for a local utility.

The reference lzip program can decompress a stream while preserving the compressed input:

lzip -dk report.txt.lz

For a TAR archive, inspect the member list before extracting it:

tar -tf source.tar.lz
tar -xf source.tar.lz -C empty-directory

GNU tar recognizes Lzip signatures and supports the --lzip filter. It invokes the external Lzip program rather than implementing the compression format itself.⁷ The Lzip project also provides tarlz, which coordinates TAR members with Lzip members for efficient creation, extraction, and recovery of .tar.lz archives.⁵

Support in Poly

Poly recognizes .lz filenames as application/x-lzip and classifies them as archives. It can store, sync, share, version, download, and find the file by name.

Poly treats the compressed stream as opaque:

• It does not decompress Lzip data or inspect a TAR archive inside it.
• It does not index decompressed text, tar member names, or member metadata.
• It does not generate a thumbnail or provide an in-app archive browser.
• The Poly agent cannot read or reason about the compressed payload.

Download and decompress the stream with a trusted local utility when you need its contents. Uploading the decompressed result separately lets Poly apply the preview and indexing pipeline appropriate to that file's own type.

Poly's extension mapping recognizes .lz, including a compound name such as backup.tar.lz because its final extension is still .lz. It does not assign this MIME type from the proposed .tlz shorthand. That suffix is also historically ambiguous because GNU tar associates .tlz with LZMA rather than Lzip.⁷

Lzip, LZMA, and XZ are not the same format

Lzip uses LZMA coding, but an Lzip file is not a raw .lzma stream or an .xz file. Their containers have different headers, trailers, checksums, and decoding rules.

Renaming one extension to another does not convert the data. Decode the original format and recompress the result when a different container is required.

Safety and recovery considerations

A small Lzip input can expand to thousands of times its compressed size. The current Internet-Draft warns that decompression can exceed a 7000-to-1 expansion ratio, so software handling untrusted files should impose output-size, memory, and time limits.⁶

The CRC and size fields detect accidental damage; they do not authenticate the sender or make the decompressed payload safe. If the output is a TAR archive, review its paths before extraction and watch for absolute paths, parent-directory references, symbolic links, and executable files.

Lzip intentionally stores no filename, timestamp, ownership, or other metadata. When metadata and multiple files matter, tar normally supplies that outer structure before Lzip compression. This also means converting document.txt.lz to another compression format requires preserving the output filename separately.

For damaged data, the companion lziprecover utility can identify members, merge matching damaged copies, and attempt recovery from corruption. Multi-member archives can limit the damage to affected members, but recovery is never a substitute for keeping another verified copy.¹