Heuristically fix UTF-8 filenames

app/src/main/java/com/sun/mail/imap/protocol/BODYSTRUCTURE.java

@@ -420,6 +420,19 @@ public class BODYSTRUCTURE implements Item {
 					", applying Exchange bug workaround");
 		    value = "";
 		}
+
+		try {
+			if (!r.supportsUtf8() &&
+					("name".equalsIgnoreCase(name) || "filename".equalsIgnoreCase(name)) &&
+					!java.nio.charset.StandardCharsets.US_ASCII.newEncoder().canEncode(value)) {
+				byte[] octets = com.sun.mail.util.ASCIIUtility.getBytes(value);
+				if (eu.faircode.email.CharsetHelper.isUTF8(octets))
+					value = new String(octets, java.nio.charset.StandardCharsets.UTF_8);
+			}
+		} catch (Throwable ex) {
+			eu.faircode.email.Log.e(ex);
+		}
+
 		list.set(name, value);
 	    } while (!r.isNextNonSpace(')'));
 	    list.combineSegments();

app/src/main/java/eu/faircode/email/CharsetHelper.java

@@ -92,7 +92,7 @@ public class CharsetHelper {
         }
     }
 
-    static boolean isUTF8(byte[] octets) {
+    public static boolean isUTF8(byte[] octets) {
         return isValid(octets, StandardCharsets.UTF_8);
     }