diff --git a/app/javascript/mastodon/features/alt_text_modal/__tests__/index-test.tsx b/app/javascript/mastodon/features/alt_text_modal/__tests__/index-test.tsx
new file mode 100644
index 0000000000..709e0ff6c2
--- /dev/null
+++ b/app/javascript/mastodon/features/alt_text_modal/__tests__/index-test.tsx
@@ -0,0 +1,91 @@
+// app/javascript/mastodon/features/alt_text_modal/__tests__/index-test.tsx
+
+import { IntlProvider } from 'react-intl';
+
+import { List, Map } from 'immutable';
+
+import { render } from '@testing-library/react';
+import { vi } from 'vitest';
+
+import type { RootState } from 'mastodon/store';
+import { useAppSelector } from 'mastodon/store';
+
+import { AltTextModal } from '../index';
+
+vi.mock('mastodon/store', () => ({
+  useAppSelector: vi.fn(),
+  useAppDispatch: () => vi.fn(),
+}));
+
+describe('<AltTextModal />', () => {
+  const mediaId = '123';
+  const handleClose = vi.fn();
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  const renderComponent = () => {
+    return render(
+      <IntlProvider locale='en' messages={{}}>
+        <AltTextModal mediaId={mediaId} onClose={handleClose} />
+      </IntlProvider>,
+    );
+  };
+
+  it('renders thumbnail upload button when video is unattached', () => {
+    vi.mocked(useAppSelector).mockImplementation(
+      (selector: (state: RootState) => unknown) => {
+        const mockState = {
+          compose: Map({
+            language: 'en',
+            media_attachments: List([
+              Map({
+                id: mediaId,
+                type: 'video',
+                unattached: true,
+                meta: Map({ focus: Map({ x: 0, y: 0 }) }),
+              }),
+            ]),
+          }),
+          accounts: Map(),
+        } as unknown as RootState;
+
+        return selector(mockState);
+      },
+    );
+
+    const { container } = renderComponent();
+
+    const uploadInput = container.querySelector('#upload-modal__thumbnail');
+    expect(uploadInput).not.toBeNull();
+  });
+
+  it('hides thumbnail upload button when video is attached', () => {
+    vi.mocked(useAppSelector).mockImplementation(
+      (selector: (state: RootState) => unknown) => {
+        const mockState = {
+          compose: Map({
+            language: 'en',
+            media_attachments: List([
+              Map({
+                id: mediaId,
+                type: 'video',
+                unattached: false,
+                meta: Map({ focus: Map({ x: 0, y: 0 }) }),
+              }),
+            ]),
+          }),
+          accounts: Map(),
+        } as unknown as RootState;
+
+        return selector(mockState);
+      },
+    );
+
+    const { container } = renderComponent();
+
+    const uploadInput = container.querySelector('#upload-modal__thumbnail');
+    expect(uploadInput).toBeNull();
+  });
+});
diff --git a/app/javascript/mastodon/features/alt_text_modal/index.tsx b/app/javascript/mastodon/features/alt_text_modal/index.tsx
index 3c54fa4724..fc00e60a51 100644
--- a/app/javascript/mastodon/features/alt_text_modal/index.tsx
+++ b/app/javascript/mastodon/features/alt_text_modal/index.tsx
@@ -283,6 +283,7 @@ export const AltTextModal = forwardRef<ModalRef, Props & Partial<RestoreProps>>(
     );
     const type = media?.get('type') as string;
     const valid = length(description) <= MAX_LENGTH;
+    const unattached = media?.get('unattached') as boolean | undefined;
 
     const handleDescriptionChange = useCallback(
       (e: React.ChangeEvent<HTMLTextAreaElement>) => {
@@ -433,7 +434,8 @@ export const AltTextModal = forwardRef<ModalRef, Props & Partial<RestoreProps>>(
               onPositionChange={handlePositionChange}
             />
 
-            {(type === 'audio' || type === 'video') && (
+            {/* This button is hidden for attached audio/video files, as they are already posted */}
+            {(type === 'audio' || type === 'video') && unattached && (
               <UploadButton
                 onSelectFile={handleThumbnailChange}
                 mimeTypes='image/jpeg,image/png,image/gif,image/heic,image/heif,image/webp,image/avif'