add multipart/mixed formatting, base64 line-wrapped encoding to MessageFormatter

Packages/MagnumOpusCore/Sources/SMTPClient/MessageFormatter.swift

@@ -70,6 +70,82 @@ public enum MessageFormatter: Sendable {
 		return formatter.string(from: date)
 	}
 
+	/// Format a multipart/mixed message with attachments.
+	public static func formatMultipart(
+		_ message: OutgoingMessage,
+		attachments: [(filename: String, mimeType: String, data: Data)]
+	) -> String {
+		let boundary = "=_MagnumOpus_\(UUID().uuidString)"
+
+		var headers: [(String, String)] = []
+		headers.append(("From", formatAddress(message.from)))
+		headers.append(("To", message.to.map(formatAddress).joined(separator: ", ")))
+
+		if !message.cc.isEmpty {
+			headers.append(("Cc", message.cc.map(formatAddress).joined(separator: ", ")))
+		}
+
+		headers.append(("Subject", message.subject))
+		headers.append(("Date", formatRFC2822Date(Date())))
+		headers.append(("Message-ID", "<\(message.messageId)>"))
+
+		if let inReplyTo = message.inReplyTo {
+			headers.append(("In-Reply-To", "<\(inReplyTo)>"))
+		}
+		if let references = message.references {
+			headers.append(("References", references))
+		}
+
+		headers.append(("MIME-Version", "1.0"))
+		headers.append(("Content-Type", "multipart/mixed; boundary=\"\(boundary)\""))
+
+		var result = ""
+		for (name, value) in headers {
+			result += "\(name): \(value)\r\n"
+		}
+		result += "\r\n"
+
+		// Text body part
+		result += "--\(boundary)\r\n"
+		result += "Content-Type: text/plain; charset=utf-8\r\n"
+		result += "Content-Transfer-Encoding: quoted-printable\r\n"
+		result += "\r\n"
+		result += quotedPrintableEncode(message.bodyText)
+		result += "\r\n"
+
+		// Attachment parts
+		for attachment in attachments {
+			result += "--\(boundary)\r\n"
+			result += "Content-Type: \(attachment.mimeType); name=\"\(attachment.filename)\"\r\n"
+			result += "Content-Disposition: attachment; filename=\"\(attachment.filename)\"\r\n"
+			result += "Content-Transfer-Encoding: base64\r\n"
+			result += "\r\n"
+			result += base64Encode(attachment.data)
+			result += "\r\n"
+		}
+
+		// Closing boundary
+		result += "--\(boundary)--\r\n"
+
+		return result
+	}
+
+	/// Base64 encode data with line wrapping at 76 characters per RFC 2045.
+	public static func base64Encode(_ data: Data, lineLength: Int = 76) -> String {
+		let encoded = data.base64EncodedString()
+		var result = ""
+		var index = encoded.startIndex
+		while index < encoded.endIndex {
+			let end = encoded.index(index, offsetBy: lineLength, limitedBy: encoded.endIndex) ?? encoded.endIndex
+			result += String(encoded[index..<end])
+			if end < encoded.endIndex {
+				result += "\r\n"
+			}
+			index = end
+		}
+		return result
+	}
+
 	/// Encode a string using quoted-printable encoding (RFC 2045).
 	public static func quotedPrintableEncode(_ text: String) -> String {
 		var result = ""