filetype

A Go library for detecting file types by content (magic bytes) and filename. Zero dependencies outside the standard library.

Features

• Detects 30+ file formats from binary content alone
• Parses OLE2 (Compound Binary File) internal directory to distinguish .doc / .xls / .ppt
• Identifies ZIP-based subtypes: OOXML (docx/xlsx/pptx), ODF (odt/ods/odp)
• Supports WPS Office formats (wps/et/dps) via filename hint
• Falls back to net/http.DetectContentType for common MIME types
• Filename suffix fallback for ambiguous or content-undetectable formats

Installation

go get github.com/servicewall/filetype

Usage

import "github.com/servicewall/filetype"

// Detect using both filename and content (recommended)
ext := filetype.Detect("report.docx", fileBytes)
// ext == "docx"

// Detect using content only
ext := filetype.DetectByContent(fileBytes)
// ext == "pdf"

// Check specific format
if filetype.IsZip(fileBytes) {
    subtype := filetype.DetectZipSubtype(fileBytes)
}

if filetype.IsOLE2(fileBytes) {
    subtype := filetype.DetectOLE2Type(fileBytes)
}

Supported Formats

Category	Extensions
Office (OOXML)	docx, xlsx, pptx
Office (OLE2)	doc, xls, ppt
Office (ODF)	odt, ods, odp
WPS Office	wps, et, dps
Documents	pdf, rtf
Images	jpg, png, gif, bmp, webp, tif, pcx
Archives	zip, gz, tar, rar, 7z, xz
Markup	html, xml
Text	txt, csv

API

Detect(filename string, body []byte) string

Primary detection function. Uses content-based detection with filename as fallback for ambiguous formats (WPS Office, unrecognized OLE2). Returns the file extension (e.g. "pdf", "docx") or "" if unknown.

DetectByContent(content []byte) string

Detects file type purely from content bytes, without any filename hint.

IsZip(data []byte) bool

Checks if data starts with the PK (ZIP) magic bytes.

DetectZipSubtype(data []byte) string

Inspects ZIP content to identify OOXML, ODF, or plain zip.

IsOLE2(data []byte) bool

Checks if data starts with the OLE2 magic signature.

DetectOLE2Type(data []byte) string

Parses OLE2 directory entries to identify doc/xls/ppt.

Detection Priority

1. WPS Office filename extensions (wps/et/dps) with matching OLE2/ZIP content
2. ZIP magic bytes -> OOXML / ODF / plain zip
3. OLE2 magic bytes -> parse directory for doc/xls/ppt
4. Custom magic byte table (RTF, 7z, XZ, TIFF, PCX)
5. TAR magic at offset 257
6. net/http.DetectContentType MIME-based detection
7. Filename suffix fallback (txt, csv, htm, xhtml, jpeg, tiff, tgz, xml)

License

MIT