Output json when receiving messages from dbus and --json parameter is given

Fixes #138

src/main/java/org/asamk/signal/json/JsonAttachment.java

@@ -0,0 +1,31 @@
+package org.asamk.signal.json;
+
+import org.whispersystems.signalservice.api.messages.SignalServiceAttachment;
+import org.whispersystems.signalservice.api.messages.SignalServiceAttachmentPointer;
+
+class JsonAttachment {
+
+    String contentType;
+    String filename;
+    String id;
+    int size;
+
+    JsonAttachment(SignalServiceAttachment attachment) {
+        this.contentType = attachment.getContentType();
+
+        final SignalServiceAttachmentPointer pointer = attachment.asPointer();
+        if (attachment.isPointer()) {
+            this.id = String.valueOf(pointer.getId());
+            if (pointer.getFileName().isPresent()) {
+                this.filename = pointer.getFileName().get();
+            }
+            if (pointer.getSize().isPresent()) {
+                this.size = pointer.getSize().get();
+            }
+        }
+    }
+
+    JsonAttachment(String filename) {
+        this.filename = filename;
+    }
+}