compose [nfc]: Split abstract base class out from _AttachFileButton

The other upload buttons will extend this new base class too.
Prompted by Greg's review:
  zulip#70 (comment)

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -280,41 +280,63 @@ Future<void> _uploadFiles({
   }
 }
 
-class _AttachFileButton extends StatelessWidget {
-  const _AttachFileButton({required this.contentController, required this.contentFocusNode});
+abstract class _AttachUploadsButton extends StatelessWidget {
+  const _AttachUploadsButton({required this.contentController, required this.contentFocusNode});
 
   final ContentTextEditingController contentController;
   final FocusNode contentFocusNode;
 
+  IconData get icon;
+  Future<Iterable<_File>?> getFiles(BuildContext context);
+
   void _handlePress(BuildContext context) async {
+    final files = await getFiles(context);
+    if (files == null) {
+      return; // Nothing to do (getFiles handles user feedback)
+    }
+
+    if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
+    else {
+      return;
+    }
+
+    await _uploadFiles(
+      context: context,
+      contentController: contentController,
+      contentFocusNode: contentFocusNode,
+      files: files);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return IconButton(icon: Icon(icon), onPressed: () => _handlePress(context));
+  }
+}
+
+class _AttachFileButton extends _AttachUploadsButton {
+  const _AttachFileButton({required super.contentController, required super.contentFocusNode});
+
+  @override
+  IconData get icon => Icons.attach_file;
+
+  @override
+  Future<Iterable<_File>?> getFiles(BuildContext context) async {
     FilePickerResult? result;
     try {
       result = await FilePicker.platform.pickFiles(allowMultiple: true, withReadStream: true);
     } catch (e) {
       // TODO(i18n)
       showErrorDialog(context: context, title: 'Error', message: e.toString());
-      return;
+      return null;
     }
     if (result == null) {
-      return; // User cancelled; do nothing
-    }
-
-    if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
-    else {
-      return;
+      return null; // User cancelled; do nothing
     }
 
-    final Iterable<_File> files = result.files.map((f) {
+    return result.files.map((f) {
       assert(f.readStream != null);  // We passed `withReadStream: true` to pickFiles.
       return _File(content: f.readStream!, length: f.size, filename: f.name);
     });
-    await _uploadFiles(context: context, contentController: contentController, contentFocusNode: contentFocusNode,
-      files: files);
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    return IconButton(icon: const Icon(Icons.attach_file), onPressed: () => _handlePress(context));
   }
 }