Polish concurrent sideload PR

- Add CHANGELOG entry for concurrent sideload behavior change
- Remove pauseItem from ActionCreators type (no thunk calls it)
- Simplify first concurrent sideload test to use fake parentId
  like the other tests in the same describe block

Author: adamsilverstein

packages/upload-media/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Enhancement
+
+- Remove sideload upload serialization: thumbnail uploads now run concurrently, governed by `maxConcurrentUploads` instead of being queued one-at-a-time per attachment ([#75257](https://github.com/WordPress/gutenberg/pull/75257)).
+
 ## 0.26.0 (2026-03-04)
 
 ## 0.25.0 (2026-02-18)

packages/upload-media/src/store/private-actions.ts

@@ -61,7 +61,6 @@ type ActionCreators = {
 	addItem: typeof addItem;
 	addSideloadItem: typeof addSideloadItem;
 	removeItem: typeof removeItem;
-	pauseItem: typeof pauseItem;
 	prepareItem: typeof prepareItem;
 	processItem: typeof processItem;
 	finishOperation: typeof finishOperation;

packages/upload-media/src/store/test/actions.ts

@@ -343,24 +343,19 @@ describe( 'actions', () => {
 				mediaSideload: jest.fn(),
 			} );
 
-			// Add a parent item first.
-			unlock( registry.dispatch( uploadStore ) ).addItem( {
-				file: jpegFile,
-			} );
-			const parentItem = unlock(
-				registry.select( uploadStore )
-			).getAllItems()[ 0 ];
+			// Use a fake parentId so we only test sideload scheduling.
+			const fakeParentId = 'fake-parent-id';
 
 			// Add two sideload items targeting the same post.
 			unlock( registry.dispatch( uploadStore ) ).addSideloadItem( {
 				file: jpegFile,
-				parentId: parentItem.id,
+				parentId: fakeParentId,
 				additionalData: { post: 100, image_size: 'thumbnail' },
 				operations: [ OperationType.Upload ],
 			} );
 			unlock( registry.dispatch( uploadStore ) ).addSideloadItem( {
 				file: jpegFile,
-				parentId: parentItem.id,
+				parentId: fakeParentId,
 				additionalData: { post: 100, image_size: 'medium' },
 				operations: [ OperationType.Upload ],
 			} );
@@ -372,7 +367,7 @@ describe( 'actions', () => {
 				registry.select( uploadStore )
 			).getAllItems();
 			const sideloadItems = items.filter(
-				( item ) => item.parentId === parentItem.id
+				( item ) => item.parentId === fakeParentId
 			);
 
 			// Neither sideload item should be paused.