Validate story attachment MIME type and fix D-Bus stub

- Reject non-image/video attachments before uploading, since stories
  only support image and video content
- Change DbusManagerImpl.sendStory to throw UnsupportedOperationException
  to match the pattern used by all other unimplemented D-Bus methods

lib/src/main/java/org/asamk/signal/manager/internal/ManagerImpl.java

@@ -835,6 +835,12 @@ public class ManagerImpl implements Manager {
             String attachment,
             boolean allowsReplies
     ) throws IOException, AttachmentInvalidException {
+        final var file = new File(attachment);
+        final var mimeType = MimeUtils.getFileMimeType(file);
+        if (mimeType.isEmpty() || (!mimeType.get().startsWith("image/") && !mimeType.get().startsWith("video/"))) {
+            throw new AttachmentInvalidException(attachment,
+                    new IOException("Stories only support image and video attachments"));
+        }
         final var uploadedAttachment = context.getAttachmentHelper().uploadAttachment(attachment);
         final var storyMessage = SignalServiceStoryMessage.forFileAttachment(account.getProfileKey().serialize(),
                 null,

src/main/java/org/asamk/signal/dbus/DbusManagerImpl.java

@@ -544,7 +544,7 @@ public class DbusManagerImpl implements Manager {
 
     @Override
     public SendMessageResults sendStory(String attachment, boolean allowsReplies) {
-        return new SendMessageResults(0, Map.of());
+        throw new UnsupportedOperationException();
     }
 
     @Override