### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/6984
1. Markdown parser supports get pictures
2. For Native, when handling Markdown, it will handle images
3. improve merge and
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
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Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>