fix(attachment:filename): fix the mime filename header

the bug happens only for filenames that contain non ascii characters like latin or german umlauts

now the content-type name and content-disposition filename behaves correctly according to the rfc2231 and rfc2047

Sources/PerfectSMTP/PerfectSMTP.swift

@@ -248,14 +248,45 @@ public class EMail {
 				print("\(data.utf8.count) bytes attached")
 			}
 			// pack it up to an MIME part
-			return "--\(boundary)\r\nContent-Type: \(mimeType); name=\"\(file)\"\r\n"
+			let filename = paramterRfc2231(something: file)
+			let name = contentTypefilenameParameter(something: file)
+			return "--\(boundary)\r\n"
+				+ "Content-Disposition: \(disposition);\r\n"
+				+ "    filename*=\(filename)\r\n"
+				+ "Content-Type: \(mimeType);\r\n"
+				+ "    name=\"\(name)\"\r\n"
 				+ "Content-Transfer-Encoding: base64\r\n"
-				+ "Content-Disposition: \(disposition); filename=\"\(file)\"\r\n\r\n\(data)\r\n"
+				+ "\r\n\(data)\r\n"
 		} catch {
 			return ""
 		}
 	}
 
+		/// https://www.ietf.org/rfc/rfc2047.txt
+		/// encode a string to url encode conform string
+		/// - parameters:
+		///   - something: a string to be encoded
+		/// - returns:
+		/// base64 but rfc2047 conform string
+		private func contentTypefilenameParameter(something: String) -> String {
+				let something = something.base64Encoded()!
+				// from rfc2047 encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
+				//   Q is "Quoted-Printable" content-transfer-encoding defined in RFC 2045
+				//   B is Base64
+				return "=?utf-8?B?\(something)?="
+		}
+
+		/// https://tools.ietf.org/html/rfc2231
+		/// encode a string to url encode conform string
+		/// - parameters:
+		///   - something: a string to be encoded
+		/// - returns:
+		/// url encoded string
+		private func paramterRfc2231(something: String) -> String {
+				let something = something.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)!
+				return "utf-8''\(something)"
+		}
+
 	/// encode a file by base64 method
 	/// - parameters:
 	///   - path: full path of the file to encode