Fix: Improve Memory Usage For Presentation (#8792)

### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/8791


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/app/presentation.py

         with slides.Presentation(BytesIO(fnm)) as presentation:
             for i, slide in enumerate(presentation.slides[from_page: to_page]):
                 try:
-                    buffered = BytesIO()
-                    slide.get_thumbnail(
-                        0.5, 0.5).save(
-                        buffered, drawing.imaging.ImageFormat.jpeg)
-                    imgs.append(Image.open(buffered))
+                    with BytesIO() as buffered:
+                        slide.get_thumbnail(
+                            0.5, 0.5).save(
+                            buffered, drawing.imaging.ImageFormat.jpeg, quality=80)
+                        imgs.append(Image.open(buffered))
                 except RuntimeError as e:
                     raise RuntimeError(f'ppt parse error at page {i+1}, original error: {str(e)}') from e
         assert len(imgs) == len(