Add command to get an attachment

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

@@ -0,0 +1,65 @@
+package org.asamk.signal.commands;
+
+import net.sourceforge.argparse4j.inf.Namespace;
+import net.sourceforge.argparse4j.inf.Subparser;
+
+import org.asamk.signal.commands.exceptions.CommandException;
+import org.asamk.signal.commands.exceptions.UnexpectedErrorException;
+import org.asamk.signal.json.JsonAttachmentData;
+import org.asamk.signal.manager.AttachmentPointer;
+import org.asamk.signal.manager.Manager;
+import org.asamk.signal.output.JsonWriter;
+import org.asamk.signal.output.OutputWriter;
+import org.asamk.signal.output.PlainTextWriter;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.util.Base64;
+
+public class AttachmentCommand implements JsonRpcLocalCommand {
+
+    @Override
+    public String getName() {
+        return "attachment";
+    }
+
+    @Override
+    public void attachToSubparser(final Subparser subparser) {
+        subparser.addArgument("--file-id")
+                .help("The ID of the attachment file.");
+        subparser.addArgument("--file-name")
+                .nargs("?")
+                .help("The name of the file.");
+        subparser.addArgument("--content-type")
+                .nargs("?")
+                .help("The content type of the file.");
+    }
+
+    @Override
+    public void handleCommand(
+            final Namespace ns,
+            final Manager m,
+            final OutputWriter outputWriter
+    ) throws CommandException {
+
+        final var id = ns.getString("file-id");
+        final var fileName = ns.getString("file-name");
+        final var contentType = ns.getString("content-type");
+
+        final var file = m.getAttachmentFile(new AttachmentPointer(id, fileName, contentType));
+
+        try {
+            final var bytes = Files.readAllBytes(file.toPath());
+            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));
+            }
+        } catch (IOException ex) {
+            throw new UnexpectedErrorException("An error occurred reading attachment file: " + file, ex);
+        }
+    }
+}