feat: add option to disable character detection

options.go

@@ -100,3 +100,15 @@ func (o disableTextConversionOption) apply(p *Parser) {
 func DisableTextConversion(disableTextConversion bool) Option {
 	return disableTextConversionOption(disableTextConversion)
 }
+
+type disableCharacterDetectionOption bool
+
+func (o disableCharacterDetectionOption) apply(p *Parser) {
+	p.disableCharacterDetection = bool(o)
+}
+
+// DisableCharacterDetection sets the disableCharacterDetection option. When true, the parser will use the
+// defined character set if it is defined in the message part.
+func DisableCharacterDetection(disableCharacterDetection bool) Option {
+	return disableCharacterDetectionOption(disableCharacterDetection)
+}

parser.go

@@ -27,6 +27,7 @@ type Parser struct {
 	customParseMediaType            CustomParseMediaType
 	stripMediaTypeInvalidCharacters bool
 	disableTextConversion           bool
+	disableCharacterDetection       bool
 }
 
 // defaultParser is a Parser with default configuration.

part.go

@@ -210,7 +210,8 @@ func (p *Part) convertFromDetectedCharset(r io.Reader, readPartErrorPolicy ReadP
 	// Restore r.
 	r = bytes.NewReader(buf)
 
-	if cs == nil || cs.Confidence < minCharsetConfidence || len(bytes.Runes(buf)) < minCharsetRuneLength {
+	if (p.parser.disableCharacterDetection && p.Charset != "") ||
+		(cs == nil || cs.Confidence < minCharsetConfidence || len(bytes.Runes(buf)) < minCharsetRuneLength) {
 		// Low confidence or not enough characters, use declared character set.
 		return p.convertFromStatedCharset(r), nil
 	}

part_test.go

@@ -1318,3 +1318,28 @@ func TestCtypeInvalidCharacters(t *testing.T) {
 
 	test.ComparePart(t, p, wantp)
 }
+
+func TestDisableCharacterDetectionPart(t *testing.T) {
+	var wantp *enmime.Part
+
+	// chardet considers this test file to be ISO-8859-1.
+	r := test.OpenTestData("parts", "chardet-detection.raw")
+	parser := enmime.NewParser(enmime.DisableCharacterDetection(true))
+	p, err := parser.ReadParts(r)
+
+	// Examine root
+	if err != nil {
+		t.Fatalf("Unexpected parse error: %+v", err)
+	}
+	if p == nil {
+		t.Fatal("Root node should not be nil")
+	}
+
+	wantp = &enmime.Part{
+		ContentType: "text/plain",
+		PartID:      "0",
+		Charset:     "utf-8",
+	}
+
+	test.ComparePart(t, p, wantp)
+}

testdata/parts/chardet-detection.raw

@@ -0,0 +1,8 @@
+Content-Type: text/plain; charset=utf-8
+Content-Transfer-Encoding: quoted-printable
+
+Loggen Sie sich ein, um die Einladung zu akzeptieren oder geben Sie den fol=
+gen1233
+
+Nachricht:
+=C3=B6o=C3=B6o