Improve filename sanitization in MIME headers

Sanitize filenames to replace invalid characters before encoding them. This prevents control and special characters from causing issues in MIME headers and file systems. The `sanitizeFilename` function ensures these characters are replaced with underscores.

msgwriter.go

@@ -273,7 +273,7 @@ func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 				mimeType = string(file.ContentType)
 			}
 			file.setHeader(HeaderContentType, fmt.Sprintf(`%s; name="%s"`, mimeType,
-				mw.encoder.Encode(mw.charset.String(), file.Name)))
+				mw.encoder.Encode(mw.charset.String(), sanitizeFilename(file.Name))))
 		}
 
 		if _, ok := file.getHeader(HeaderContentTransferEnc); !ok {
@@ -285,7 +285,7 @@ func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 
 		if file.Desc != "" {
 			if _, ok := file.getHeader(HeaderContentDescription); !ok {
-				file.setHeader(HeaderContentDescription, file.Desc)
+				file.setHeader(HeaderContentDescription, mw.encoder.Encode(mw.charset.String(), file.Desc))
 			}
 		}
 
@@ -295,12 +295,12 @@ func (mw *msgWriter) addFiles(files []*File, isAttachment bool) {
 				disposition = "attachment"
 			}
 			file.setHeader(HeaderContentDisposition, fmt.Sprintf(`%s; filename="%s"`,
-				disposition, mw.encoder.Encode(mw.charset.String(), file.Name)))
+				disposition, mw.encoder.Encode(mw.charset.String(), sanitizeFilename(file.Name))))
 		}
 
 		if !isAttachment {
 			if _, ok := file.getHeader(HeaderContentID); !ok {
-				file.setHeader(HeaderContentID, fmt.Sprintf("<%s>", file.Name))
+				file.setHeader(HeaderContentID, fmt.Sprintf("<%s>", sanitizeFilename(file.Name)))
 			}
 		}
 		if mw.depth == 0 {
@@ -498,3 +498,33 @@ func (mw *msgWriter) writeBody(writeFunc func(io.Writer) (int64, error), encodin
 		mw.bytesWritten += n
 	}
 }
+
+// sanitizeFilename sanitizes a given filename string by replacing specific unwanted characters with
+// an underscore ('_').
+//
+// This method replaces any control character and any special character that is problematic for
+// MIME headers and file systems with an underscore ('_') character.
+//
+// The following characters are replaced
+// - Any control character (US-ASCII < 32)
+// - ", /, :, <, >, ?, \, |, [DEL]
+//
+// Parameters:
+//   - input: A string of a filename that is supposed to be sanitized
+//
+// Returns:
+//   - A string representing the sanitized version of the filename
+func sanitizeFilename(input string) string {
+	var sanitized strings.Builder
+	for i := 0; i < len(input); i++ {
+		// We do not allow control characters in file names.
+		if input[i] < 32 || input[i] == 34 || input[i] == 47 || input[i] == 58 ||
+			input[i] == 60 || input[i] == 62 || input[i] == 63 || input[i] == 92 ||
+			input[i] == 124 || input[i] == 127 {
+			sanitized.WriteRune('_')
+			continue
+		}
+		sanitized.WriteByte(input[i])
+	}
+	return sanitized.String()
+}