1 files changed, 143 insertions, 0 deletions
diff --git a/modules/typesniffer/typesniffer.go b/modules/typesniffer/typesniffer.go
new file mode 100644
index 0000000..6aec5c2
--- /dev/null
+++ b/modules/typesniffer/typesniffer.go
@@ -0,0 +1,143 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package typesniffer
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net/http"
+	"regexp"
+	"strings"
+
+	"code.gitea.io/gitea/modules/util"
+)
+
+// Use at most this many bytes to determine Content Type.
+const sniffLen = 1024
+
+const (
+	// SvgMimeType MIME type of SVG images.
+	SvgMimeType = "image/svg+xml"
+	// ApplicationOctetStream MIME type of binary files.
+	ApplicationOctetStream = "application/octet-stream"
+)
+
+var (
+	svgComment       = regexp.MustCompile(`(?s)<!--.*?-->`)
+	svgTagRegex      = regexp.MustCompile(`(?si)\A\s*(?:(<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg\b`)
+	svgTagInXMLRegex = regexp.MustCompile(`(?si)\A<\?xml\b.*?\?>\s*(?:(<!DOCTYPE\s+svg([\s:]+.*?>|>))\s*)*<svg\b`)
+)
+
+// SniffedType contains information about a blobs type.
+type SniffedType struct {
+	contentType string
+}
+
+// IsText etects if content format is plain text.
+func (ct SniffedType) IsText() bool {
+	return strings.Contains(ct.contentType, "text/")
+}
+
+// IsImage detects if data is an image format
+func (ct SniffedType) IsImage() bool {
+	return strings.Contains(ct.contentType, "image/")
+}
+
+// IsSvgImage detects if data is an SVG image format
+func (ct SniffedType) IsSvgImage() bool {
+	return strings.Contains(ct.contentType, SvgMimeType)
+}
+
+// IsPDF detects if data is a PDF format
+func (ct SniffedType) IsPDF() bool {
+	return strings.Contains(ct.contentType, "application/pdf")
+}
+
+// IsVideo detects if data is an video format
+func (ct SniffedType) IsVideo() bool {
+	return strings.Contains(ct.contentType, "video/")
+}
+
+// IsAudio detects if data is an video format
+func (ct SniffedType) IsAudio() bool {
+	return strings.Contains(ct.contentType, "audio/")
+}
+
+// IsRepresentableAsText returns true if file content can be represented as
+// plain text or is empty.
+func (ct SniffedType) IsRepresentableAsText() bool {
+	return ct.IsText() || ct.IsSvgImage()
+}
+
+// IsBrowsableBinaryType returns whether a non-text type can be displayed in a browser
+func (ct SniffedType) IsBrowsableBinaryType() bool {
+	return ct.IsImage() || ct.IsSvgImage() || ct.IsPDF() || ct.IsVideo() || ct.IsAudio()
+}
+
+// GetMimeType returns the mime type
+func (ct SniffedType) GetMimeType() string {
+	return strings.SplitN(ct.contentType, ";", 2)[0]
+}
+
+// DetectContentType extends http.DetectContentType with more content types. Defaults to text/unknown if input is empty.
+func DetectContentType(data []byte) SniffedType {
+	if len(data) == 0 {
+		return SniffedType{"text/unknown"}
+	}
+
+	ct := http.DetectContentType(data)
+
+	if len(data) > sniffLen {
+		data = data[:sniffLen]
+	}
+
+	// SVG is unsupported by http.DetectContentType, https://github.com/golang/go/issues/15888
+
+	detectByHTML := strings.Contains(ct, "text/plain") || strings.Contains(ct, "text/html")
+	detectByXML := strings.Contains(ct, "text/xml")
+	if detectByHTML || detectByXML {
+		dataProcessed := svgComment.ReplaceAll(data, nil)
+		dataProcessed = bytes.TrimSpace(dataProcessed)
+		if detectByHTML && svgTagRegex.Match(dataProcessed) ||
+			detectByXML && svgTagInXMLRegex.Match(dataProcessed) {
+			ct = SvgMimeType
+		}
+	}
+
+	if strings.HasPrefix(ct, "audio/") && bytes.HasPrefix(data, []byte("ID3")) {
+		// The MP3 detection is quite inaccurate, any content with "ID3" prefix will result in "audio/mpeg".
+		// So remove the "ID3" prefix and detect again, if result is text, then it must be text content.
+		// This works especially because audio files contain many unprintable/invalid characters like `0x00`
+		ct2 := http.DetectContentType(data[3:])
+		if strings.HasPrefix(ct2, "text/") {
+			ct = ct2
+		}
+	}
+
+	if ct == "application/ogg" {
+		dataHead := data
+		if len(dataHead) > 256 {
+			dataHead = dataHead[:256] // only need to do a quick check for the file header
+		}
+		if bytes.Contains(dataHead, []byte("theora")) || bytes.Contains(dataHead, []byte("dirac")) {
+			ct = "video/ogg" // ogg is only used for some video formats, and it's not popular
+		} else {
+			ct = "audio/ogg" // for most cases, it is used as an audio container
+		}
+	}
+	return SniffedType{ct}
+}
+
+// DetectContentTypeFromReader guesses the content type contained in the reader.
+func DetectContentTypeFromReader(r io.Reader) (SniffedType, error) {
+	buf := make([]byte, sniffLen)
+	n, err := util.ReadAtMost(r, buf)
+	if err != nil {
+		return SniffedType{}, fmt.Errorf("DetectContentTypeFromReader io error: %w", err)
+	}
+	buf = buf[:n]
+
+	return DetectContentType(buf), nil
+}