add support for sending media

Still needs camera support.

lib/controllers/attachment.dart

@@ -0,0 +1,41 @@
+import "package:file_selector/file_selector.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/client.dart";
+import "package:nexus/controllers/rooms.dart";
+import "package:nexus/models/content/content.dart";
+import "package:nexus/models/content/message.dart";
+import "package:path/path.dart";
+
+class AttachmentController extends Notifier<(String, MessageContent?)?> {
+  final String roomId;
+  AttachmentController(this.roomId);
+
+  @override
+  Null build() => null;
+
+  Future<void> add(XFile file) async {
+    final filename = basename(file.path);
+    state = (filename, null);
+
+    final room = ref.watch(
+      RoomsController.provider.select((value) => value[roomId]),
+    );
+
+    final content = await ref
+        .watch(ClientController.provider.notifier)
+        .uploadMedia(
+          .new(
+            path: file.path,
+            encrypted:
+                room?.state[EventType.encryption.name]?.isNotEmpty == true,
+          ),
+        );
+
+    state = (filename, content);
+  }
+
+  static final provider = NotifierProvider.family
+      .autoDispose<AttachmentController, (String, MessageContent?)?, String>(
+        AttachmentController.new,
+      );
+}