allow pasting attachments from keyboard

Closes #55

lib/widgets/composer/paste_files_action.dart

@@ -0,0 +1,12 @@
+import "package:material_ui/material_ui.dart";
+
+/// Overrides a text field's paste, to paste files and images with
+/// [pasteFiles], falling back to pasting text if it returns false.
+class PasteFilesAction(final Future<bool> Function() pasteFiles)
+    extends Action<PasteTextIntent> {
+  @override
+  Future<void> invoke(PasteTextIntent intent) async {
+    // callingAction is the text field's own paste action, which this overrides
+    if (!await pasteFiles()) callingAction?.invoke(intent);
+  }
+}