feat(knowledge): 添加知识条目文件预览和替换功能

- 后端实现知识条目文件预览接口，支持多种文件类型在线预览
- 后端实现知识条目文件替换功能，保留原有文件管理逻辑
- 前端新增文件预览模态框组件，支持文本、图片、音视频预览
- 前端知识条目编辑器添加文件替换上传功能
- 前端优化文件内容截断预览逻辑，统一使用工具函数处理
- 前端修复 PUT 请求中 FormData 处理问题，确保文件上传正常工作
- 新增文件预览相关工具函数和常量配置

frontend/src/pages/KnowledgeManagement/components/KnowledgeItemEditor.tsx

@@ -4,6 +4,7 @@ import { UploadOutlined } from "@ant-design/icons";
 import dayjs from "dayjs";
 import {
   downloadKnowledgeItemFileUsingGet,
+  replaceKnowledgeItemFileUsingPut,
   updateKnowledgeItemByIdUsingPut,
   uploadKnowledgeItemsUsingPost,
 } from "../knowledge-management.api";
@@ -44,6 +45,8 @@ export default function KnowledgeItemEditor({
   const [form] = Form.useForm();
   const [tagOptions, setTagOptions] = useState<{ label: string; value: string }[]>([]);
   const [fileList, setFileList] = useState<UploadFile[]>([]);
+  const [replaceFileList, setReplaceFileList] = useState<UploadFile[]>([]);
+  const [titleBeforeReplace, setTitleBeforeReplace] = useState<string | null>(null);
   const isMultiFile = fileList.length > 1;
   const isFileItem =
     data?.contentType === KnowledgeContentType.FILE ||
@@ -89,16 +92,21 @@ export default function KnowledgeItemEditor({
           tags: data.tags?.map((tag) => tag.name) || [],
           metadata: data.metadata,
         });
+        setTitleBeforeReplace(null);
       } else {
         form.resetFields();
         form.setFieldsValue({
           status: KnowledgeStatusType.DRAFT,
           tags: [],
         });
+        setTitleBeforeReplace(null);
       }
       setFileList([]);
+      setReplaceFileList([]);
     } else {
       setFileList([]);
+      setReplaceFileList([]);
+      setTitleBeforeReplace(null);
     }
   }, [open, data, form]);
 
@@ -138,6 +146,32 @@ export default function KnowledgeItemEditor({
     return true;
   };
 
+  const handleReplaceFileBeforeUpload = (file: File) => {
+    if (!titleBeforeReplace) {
+      setTitleBeforeReplace(form.getFieldValue("title") || null);
+    }
+    setReplaceFileList([
+      {
+        uid: `${Date.now()}-${file.name}`,
+        name: file.name,
+        status: "done",
+        originFileObj: file,
+      },
+    ]);
+    form.setFieldsValue({ title: stripFileExtension(file.name) });
+    message.success("已选择替换文件，提交后生效");
+    return false;
+  };
+
+  const handleReplaceFileRemove = (removedFile: UploadFile) => {
+    setReplaceFileList((prev) => prev.filter((file) => file.uid !== removedFile.uid));
+    if (titleBeforeReplace !== null) {
+      form.setFieldsValue({ title: titleBeforeReplace || undefined });
+      setTitleBeforeReplace(null);
+    }
+    return true;
+  };
+
   const handleDownloadFile = async () => {
     if (!data?.id) {
       return;
@@ -166,13 +200,26 @@ export default function KnowledgeItemEditor({
       }
 
       if (data?.id) {
-        const payload = {
+        const payload: Record<string, unknown> = {
           ...values,
           validFrom,
           validTo,
           tags: values.tags || [],
         };
+        if (replaceFileList.length > 0) {
+          delete payload.title;
+        }
         await updateKnowledgeItemByIdUsingPut(setId, data.id, payload);
+        if (replaceFileList.length > 0) {
+          const formData = new FormData();
+          const replaceFile = replaceFileList[0]?.originFileObj as File | undefined;
+          if (!replaceFile) {
+            message.warning("请先选择要替换的文件");
+            return;
+          }
+          formData.append("file", replaceFile);
+          await replaceKnowledgeItemFileUsingPut(setId, data.id, formData);
+        }
         message.success("知识条目更新成功");
       } else {
         if (fileList.length === 0) {
@@ -224,6 +271,8 @@ export default function KnowledgeItemEditor({
 
       form.resetFields();
       setFileList([]);
+      setReplaceFileList([]);
+      setTitleBeforeReplace(null);
       onSuccess();
     } catch {
       message.error("操作失败，请重试");
@@ -289,6 +338,20 @@ export default function KnowledgeItemEditor({
             </div>
           </Form.Item>
         )}
+        {data?.id && isFileItem && !readOnly && (
+          <Form.Item label="替换文件">
+            <Upload
+              beforeUpload={handleReplaceFileBeforeUpload}
+              fileList={replaceFileList}
+              onRemove={handleReplaceFileRemove}
+              showUploadList={{ showPreviewIcon: false }}
+              maxCount={1}
+            >
+              <Button icon={<UploadOutlined />}>选择文件</Button>
+            </Upload>
+            <div className="text-xs text-gray-500 mt-1">仅支持单文件，替换后标题自动更新</div>
+          </Form.Item>
+        )}
         <div className="grid grid-cols-2 gap-4">
           <Form.Item label="内容类型">
             <Input value={contentTypeLabel} disabled />