doc: clarify image format attribute in extraction guides

- Add format field examples
- Note format normalization rules
- Explain when format is omitted
- Update both EN and ZH guides

Author: danielaskdd

doc/doc-extraction-guide.md

@@ -54,6 +54,9 @@ The resulting `_blocks.jsonl` is the foundational input for the auditing system.
 - **Images**: `<drawing id="..." name="..." path="..." format="..." />`
   - Embedded image (`r:embed`): exported to `<blocks_stem>.image/`, `path` stores relative file path
   - Linked image (`r:link`): not downloaded, `path` stores original external URL
+  - `format`: normalized image format derived from content-type/file extension, common values include
+    `png`, `jpeg`, `emf`, `wmf`, `gif`, `bmp`, `tiff`, `webp`, `svg`
+  - `format` is omitted when format cannot be determined or when drawing has no image blip (e.g., chart shape)
 - **Formulas**: OMML → LaTeX, wrapped in `<equation>...</equation>`
 - **Super/Subscripts**: `<sup>...</sup>` / `<sub>...</sub>`
 

doc/docx-extraction-guide-zh.md

@@ -54,6 +54,9 @@
 - **图片**：`<drawing id="..." name="..." path="..." format="..." />`
   - 内嵌图片（`r:embed`）：导出到 `<blocks_stem>.image/`，`path` 写相对路径
   - 外链图片（`r:link`）：不下载，`path` 写原始外链 URL
+  - `format`：由 content-type/文件扩展名归一化得到，常见取值包括
+    `png`、`jpeg`、`emf`、`wmf`、`gif`、`bmp`、`tiff`、`webp`、`svg`
+  - 若无法判定格式，或该 drawing 不包含图片 blip（如图表形状），则省略 `format` 属性
 - **公式**：OMML → LaTeX，包裹 `<equation>...</equation>`
 - **上下标**：`<sup>...</sup>` / `<sub>...</sub>`