ZIP Archive MIME Type

What does application/zip mean?

The application/zip media type identifies a ZIP archive, normally named with the .zip extension. A ZIP file can bundle files and directories into one container. Each member may be compressed independently, stored without compression, encrypted, or digitally signed.¹

ZIP is an archive format as well as a compression framework. It does not require every member to be compressed, and one archive can use different methods for different members. DEFLATE is the method most ZIP applications use by default.¹

Where did ZIP come from?

PKWARE first published the ZIP specification in 1989. It defines a portable format for aggregating, compressing, and encrypting files in one interoperable container. The company continues to maintain the .ZIP File Format Specification, commonly called APPNOTE.¹

The application/zip registration dates to July 1993. It defines a binary media type with no required or optional parameters and points to PKWARE's format specification.² ISO/IEC later standardized a deliberately limited ZIP profile as ISO/IEC 21320-1:2015. That document-container standard references APPNOTE 6.3.3 and does not redefine every feature in the broader ZIP ecosystem.³

How do you identify a ZIP archive?

Most non-empty ZIP archives begin with 50 4B 03 04, the little-endian representation of the local-file-header signature 0x04034B50. The PK bytes refer to PKWARE. An empty archive may instead begin with the end-of-central-directory signature 50 4B 05 06.¹

50 4B 03 04 ...  local file header
50 4B 01 02 ...  central directory entry
50 4B 05 06 ...  end of central directory

These record signatures are more useful than a filename extension, but checking only the first four bytes is not complete validation. A conforming archive must contain one end-of-central-directory record. Each stored member has a local header and a corresponding entry in the central directory.¹

ZIP also supports split and self-extracting archives. Those forms may not look like an ordinary single-file ZIP at the start, so a robust reader should locate and validate the directory rather than rely on one opening signature.¹

How does ZIP compression work?

A ZIP archive keeps a separate record for each member, including its name, compression method, CRC-32, compressed size, and uncompressed size. The central directory collects those records near the end of the file so tools can list members without decompressing all of them.¹

Per-file compression makes random extraction convenient. It can be less efficient than solid archive formats when many small files share repeated content, because a normal ZIP compressor cannot carry one member's compression history into the next. Already-compressed formats such as JPEG, MP4, and many PDFs often shrink little when placed in ZIP. Microsoft similarly notes that a ZIP of JPEG images may be about the same size as the originals.⁴

Classic ZIP fields limit sizes and member counts. ZIP64 adds larger fields for archives beyond the older 4 GiB and 65,535-entry boundaries. A tool can support ordinary ZIP while still rejecting ZIP64, a newer compression method, or a particular encryption scheme, so the .zip extension alone does not guarantee full interoperability.¹

How do you open an application/zip file?

Web browsers normally download a response labeled application/zip; they do not provide a standard in-page archive viewer. Desktop operating systems have broad built-in support:

• Windows File Explorer can create compressed folders, list their contents, and extract all or selected members.⁴
• macOS Finder creates a .zip from its Compress command and expands one when you double-click it.⁵
• 7-Zip and other archive utilities provide listing, testing, selective extraction, and support for additional ZIP methods.

Inspect an unfamiliar archive before extracting it. With common command-line tools:

unzip -l package.zip
unzip -t package.zip
unzip package.zip -d empty-directory

The list command shows stored paths, and the test command checks whether the tool can read member data and verify its CRC values. A passing CRC test detects accidental corruption. It is not proof that the contents are trustworthy or that the archive came from the claimed sender.

Support in Poly

Poly recognizes .zip filenames as application/zip and classifies them as archives. It can store, sync, share, version, download, and find the archive by name.

Poly currently treats the archive 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 member browser.
• Search can match the .zip file's name and ordinary Poly properties, but not text packed inside it.
• The Poly agent cannot read or reason about the archived files.

Download and extract the archive with a trusted utility when you need its contents. If you add the extracted members to Poly separately, each file can use the preview, metadata, and indexing pipeline for its own format.

Is ZIP the same as Gzip, TAR, or 7z?

No. These formats solve overlapping problems with different structures:

Converting between archive formats means extracting the members and building a new archive. Renaming backup.zip to backup.tar or backup.7z changes only the filename. Repacking can also lose permissions, links, timestamps, comments, encryption, or other metadata that the destination does not represent in the same way.

Security and privacy considerations

Treat every archive as untrusted input. The original IANA registration warns that extraction can overwrite existing files and recommends extracting into an empty directory rather than a filesystem root.² Modern extractors should also reject absolute paths and parent-directory traversal, but you should not assume every tool does so correctly. Python's standard-library documentation, for example, makes path validation the caller's responsibility for its archive-path API.⁶

Review executable files and symbolic links before opening anything you extracted. Scan the expanded files, keep the archive tool patched, and do not use an online converter for confidential or encrypted archives.

ZIP can encrypt individual members and, with stronger extensions, the central directory.¹ Encryption compatibility varies across tools, and a password does not authenticate the sender. For sensitive transfers, use a well-supported encryption profile, a strong unique password delivered through a separate channel, and an authenticated mechanism such as a digital signature when provenance matters.

Member names, comments, timestamps, and platform-specific attributes can also disclose information. Even when file data is encrypted, the exact metadata hidden depends on the encryption profile and software used. Inspect the archive with the same privacy care you would apply to the files inside it.