Prefer first text/html part

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

@@ -1038,10 +1038,13 @@ public class MessageHelper {
                         TextUtils.isEmpty(filename) &&
                         ((parts.plain == null && part.isMimeType("text/plain")) ||
                                 (parts.html == null && part.isMimeType("text/html")))) {
-                    if (part.isMimeType("text/plain"))
+                    if (part.isMimeType("text/plain")) {
+                        if (parts.plain == null)
                             parts.plain = part;
-                    else
+                    } else {
+                        if (parts.html == null)
                             parts.html = part;
+                    }
                 } else {
                     AttachmentPart apart = new AttachmentPart();
                     apart.disposition = disposition;