Correctly handle alternative text formats

FAQ.md

@@ -38,7 +38,6 @@ None at this moment.
 * Automatically go to the next message on deleting a message: since the 'next' message can either be an older or a newer message this would be confusing. You can disable auto closing in the advanced options and use the bottom navigation bar instead.
 * Rich text editor: besides that very few people would use this on a small mobile device, Android doesn't support a rich text editor and most rich text editor open source projects are abandoned.
 * Widget to read e-mail: widgets can have limited user interaction only, so a widget to read e-mail would not be very useful. Moreover, it would be not very useful to duplicate functions which are already available in the app.
-* Calendar events: opening the attached calendar file should open the related calendar app.
 * Executing filter rules: filter rules should be executed on the server because a battery powered device with possibly an unstable internet connection is not suitable for this.
 * Badge count: there is no standard Android API for this and third party solutions might stop working anytime. For example *ShortcutBadger* [has lots of problems](https://github.com/leolin310148/ShortcutBadger/issues). You can use the provided widget instead.
 * Switch language: although it is possible to change the language of an app, Android is not designed for this. Better fix the translation in your language if needed, see [this FAQ](#user-content-faq26) about how to.

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

@@ -519,7 +519,7 @@ public class MessageHelper {
     }
 
     private static String getHtml(Part part) throws MessagingException, IOException {
-        if (part.isMimeType("text/*")) {
+        if (part.isMimeType("text/plain") || part.isMimeType("text/html")) {
             String s;
             try {
                 Object content = part.getContent();
@@ -654,7 +654,7 @@ public class MessageHelper {
             }
 
             if (Part.ATTACHMENT.equalsIgnoreCase(disposition) ||
-                    part.isMimeType("image/*") ||
+                    !(part.isMimeType("text/plain") || part.isMimeType("text/html")) ||
                     !TextUtils.isEmpty(filename)) {
                 ContentType ct = new ContentType(part.getContentType());
                 String[] cid = part.getHeader("Content-ID");
@@ -668,10 +668,12 @@ public class MessageHelper {
                 attachment.encryption = (pgp ? EntityAttachment.PGP_MESSAGE : null);
                 attachment.part = part;
 
+                if (TextUtils.isEmpty(attachment.name) && "text/calendar".equals(attachment.type))
+                    attachment.name = "invite.ics";
+
                 // Try to guess a better content type
                 // Sometimes PDF files are sent using the wrong type
-                if ("application/octet-stream".equals(attachment.type) ||
-                        "message/disposition-notification".equals(attachment.type)) {
+                if ("application/octet-stream".equals(attachment.type)) {
                     String extension = Helper.getExtension(attachment.name);
                     if (extension != null) {
                         String type = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension.toLowerCase());