Attempt to fix broken encodings

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

@@ -35,6 +35,7 @@ import java.io.OutputStream;
 import java.io.UnsupportedEncodingException;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Date;
 import java.util.Enumeration;
 import java.util.List;
@@ -58,6 +59,7 @@ import javax.mail.internet.InternetAddress;
 import javax.mail.internet.MimeBodyPart;
 import javax.mail.internet.MimeMessage;
 import javax.mail.internet.MimeMultipart;
+import javax.mail.internet.MimePart;
 import javax.mail.internet.MimeUtility;
 import javax.mail.internet.ParseException;
 
@@ -70,6 +72,13 @@ public class MessageHelper {
 
     static final int ATTACHMENT_BUFFER_SIZE = 8192; // bytes
 
+    private static final List<String> ENCODINGS = Arrays.asList(
+            "base64",
+            "quoted-printable",
+            "uuencode", "x-uuencode", "x-uue",
+            "binary", "7bit", "8bit"
+    );
+
     static Properties getSessionProperties(int auth_type, String realm, boolean insecure) {
         Properties props = new Properties();
 
@@ -595,6 +604,8 @@ public class MessageHelper {
             Part part = (html == null ? plain : html);
 
             try {
+                fixEncoding((MimePart) part);
+
                 Object content = part.getContent();
                 if (content instanceof String)
                     result = (String) content;
@@ -633,6 +644,33 @@ public class MessageHelper {
             return result;
         }
 
+        private void fixEncoding(MimePart part) throws MessagingException {
+            String mencoding = part.getEncoding();
+            if (mencoding == null || ENCODINGS.contains(mencoding))
+                return;
+
+            String cte = part.getHeader("Content-Transfer-Encoding")[0];
+            Log.i("Attempting to fix encoding=" + cte);
+
+            for (String encoding : ENCODINGS) {
+                boolean contains = true;
+                for (String pencoding : encoding.split("-"))
+                    contains = (contains && mencoding.toLowerCase().contains(pencoding));
+
+                if (contains) {
+                    String warning = "Changed invalid encoding " + cte + " to " + encoding;
+                    Log.w(warning);
+                    warnings.add(warning);
+                    part.setHeader("Content-Transfer-Encoding", encoding);
+                    return;
+                }
+            }
+
+            String warning = "Unfixable encoding " + cte;
+            Log.w(warning);
+            warnings.add(warning);
+        }
+
         List<EntityAttachment> getAttachments() throws MessagingException {
             List<EntityAttachment> result = new ArrayList<>();