Use JSON serializer to serialize binary data

Serializing the stream is better for memory handling than
loading the whole thing into the file.

src/main/java/org/asamk/signal/commands/GetAttachmentCommand.java

@@ -47,17 +47,15 @@ public class GetAttachmentCommand implements JsonRpcLocalCommand {
         final var id = ns.getString("id");
 
         try(InputStream attachment = m.retrieveAttachment(id)) {
-
+            if (outputWriter instanceof PlainTextWriter writer) {
                 final var bytes = attachment.readAllBytes();
                 final var base64 = Base64.getEncoder().encodeToString(bytes);
-
-            if (outputWriter instanceof PlainTextWriter writer) {
                 writer.println(base64);
             } else if (outputWriter instanceof JsonWriter writer) {
-                writer.write(new JsonAttachmentData(base64));
+                writer.write(new JsonAttachmentData(attachment));
             }
         } catch (FileNotFoundException ex) {
-            throw new UserErrorException("Could not find attachment with ID: " + id);
+            throw new UserErrorException("Could not find attachment with ID: " + id, ex);
         } catch (IOException ex) {
             throw new UnexpectedErrorException("An error occurred reading attachment: " + id, ex);
         }

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

@@ -1,3 +1,9 @@
 package org.asamk.signal.json;
 
-public record JsonAttachmentData(String data) {}
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
+
+import java.io.InputStream;
+
+public record JsonAttachmentData(
+        @JsonSerialize(using=JsonStreamSerializer.class) InputStream data
+) {}

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

@@ -0,0 +1,20 @@
+package org.asamk.signal.json;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.JsonSerializer;
+import com.fasterxml.jackson.databind.SerializerProvider;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+public class JsonStreamSerializer extends JsonSerializer<InputStream> {
+
+    @Override
+    public void serialize(
+            final InputStream value,
+            final JsonGenerator jsonGenerator,
+            final SerializerProvider serializers
+    ) throws IOException {
+        jsonGenerator.writeBinary(value, -1);
+    }
+}