compose [nfc]: Expose uploadFiles on ComposeBoxState

This will be used soon to allow uploading files directly
via using `MessageListPageState`.

Author: rajveermalviya

lib/widgets/compose_box.dart

@@ -1869,6 +1869,20 @@ abstract class ComposeBoxState extends State<ComposeBox> {
 
   /// Switch the compose box back to regular non-edit mode, with no content.
   void endEditInteraction();
+
+  /// Uploads the provided files, populating the compose box with their links.
+  ///
+  /// If any of the file is larger than maximum file size allowed by the
+  /// server, an error dialog is shown mentioning their names and actual
+  /// file sizes.
+  ///
+  /// While uploading, a placeholder link is inserted in the compose box and
+  /// it is replaced with an actual link after uploading completes
+  /// successfully.
+  ///
+  /// If there is an error while uploading a file, then an error dialog is
+  /// shown mentioning the corresponding file name.
+  Future<void> uploadFiles(Iterable<FileToUpload> files);
 }
 
 class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateMixin<ComposeBox> implements ComposeBoxState {
@@ -2021,6 +2035,15 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
     });
   }
 
+  @override
+  Future<void> uploadFiles(Iterable<FileToUpload> files) async {
+    await _uploadFiles(
+      context: context,
+      contentController: controller.content,
+      contentFocusNode: controller.contentFocusNode,
+      files: files);
+  }
+
   @override
   void onNewStore() {
     final newStore = PerAccountStoreWidget.of(context);