feat[Go] implement api/v1/thumbnails API (#15416)

### What problem does this PR solve?

As title

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality

internal/handler/document.go

@@ -18,8 +18,11 @@ package handler
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
+	"mime"
 	"net/http"
+	"path/filepath"
 	"ragflow/internal/common"
 	"ragflow/internal/entity"
 	"strconv"
@@ -31,6 +34,8 @@ import (
 	"ragflow/internal/service"
 )
 
+var IMG_BASE64_PREFIX = "data:image/png;base64,"
+
 // DocumentHandler document handler
 type DocumentHandler struct {
 	documentService *service.DocumentService
@@ -120,6 +125,58 @@ func (h *DocumentHandler) GetDocumentByID(c *gin.Context) {
 	})
 }
 
+// GetThumbnail Get thumbnails for documents.
+func (h *DocumentHandler) GetThumbnail(c *gin.Context) {
+	_, errorCode, errorMessage := GetUser(c)
+	if errorCode != common.CodeSuccess {
+		jsonError(c, errorCode, errorMessage)
+		return
+	}
+
+	id := c.Query("doc_ids")
+	if id == "" {
+		c.JSON(http.StatusBadRequest, gin.H{
+			"error": errors.New("invalid document id"),
+		})
+		return
+	}
+
+	result, err := h.documentService.GetThumbnail(id)
+	if err != nil {
+		c.JSON(http.StatusNotFound, gin.H{
+			"error": fmt.Errorf("thumbnail not found"),
+		})
+		return
+	}
+
+	if result.Thumbnail != nil && *result.Thumbnail != "" {
+		newThumbURL := fmt.Sprintf("/api/v1/documents/images/%s-%s", result.KbID, *result.Thumbnail)
+		result.Thumbnail = &newThumbURL
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"code":    common.CodeSuccess,
+		"data":    map[string]interface{}{result.ID: result.Thumbnail},
+		"message": "success",
+	})
+}
+
+// GetDocumentImage returns a document image from object storage.
+func (h *DocumentHandler) GetDocumentImage(c *gin.Context) {
+	imageID := c.Param("image_id")
+	data, err := h.documentService.GetDocumentImage(imageID)
+	if err != nil {
+		jsonError(c, common.CodeDataError, "Image not found.")
+		return
+	}
+
+	contentType := mime.TypeByExtension(strings.ToLower(filepath.Ext(imageID)))
+	if contentType == "" {
+		contentType = "image/JPEG"
+	}
+	c.Data(http.StatusOK, contentType, data)
+}
+
 // UpdateDocument update document
 // @Summary Update Document
 // @Description Update document info

internal/router/router.go

@@ -116,6 +116,9 @@ func (r *Router) Setup(engine *gin.Engine) {
 
 		// Register
 		apiNoAuth.POST("/users", r.userHandler.Register)
+
+		// Document images are embedded directly in pages and match Python's public route.
+		apiNoAuth.GET("/documents/images/:image_id", r.documentHandler.GetDocumentImage)
 	}
 
 	// Protected routes
@@ -411,6 +414,8 @@ func (r *Router) Setup(engine *gin.Engine) {
 			doc.POST("/delete_meta", r.documentHandler.DeleteMeta) // Internal API only for GO
 		}
 
+		v1.GET("/thumbnails", r.documentHandler.GetThumbnail)
+
 		// Chunk routes
 		chunk := v1.Group("/chunk")
 		{

internal/service/document.go

@@ -21,8 +21,10 @@ import (
 	"fmt"
 	"ragflow/internal/common"
 	"ragflow/internal/entity"
+	"ragflow/internal/storage"
 	"regexp"
 	"sort"
+	"strings"
 	"time"
 
 	"ragflow/internal/dao"
@@ -102,6 +104,27 @@ type DocumentResponse struct {
 	UpdatedAt       string  `json:"updated_at"`
 }
 
+type ThumbnailResponse struct {
+	ID        string  `json:"id"`
+	Thumbnail *string `json:"thumbnail,omitempty"`
+	KbID      string  `json:"kb_id"`
+}
+
+// GetDocumentImage retrieves an image object from storage.
+func (s *DocumentService) GetDocumentImage(imageID string) ([]byte, error) {
+	parts := strings.Split(imageID, "-")
+	if len(parts) != 2 || parts[0] == "" || parts[1] == "" {
+		return nil, fmt.Errorf("Image not found.")
+	}
+
+	storageImpl := storage.GetStorageFactory().GetStorage()
+	if storageImpl == nil {
+		return nil, fmt.Errorf("storage not initialized")
+	}
+
+	return storageImpl.Get(parts[0], parts[1])
+}
+
 // CreateDocument create document
 func (s *DocumentService) CreateDocument(req *CreateDocumentRequest) (*entity.Document, error) {
 	document := &entity.Document{
@@ -182,6 +205,19 @@ func (s *DocumentService) ListDocuments(page, pageSize int) ([]*DocumentResponse
 	return responses, total, nil
 }
 
+func (s *DocumentService) GetThumbnail(docID string) (*ThumbnailResponse, error) {
+	document, err := s.documentDAO.GetByID(docID)
+	if err != nil {
+		return nil, err
+	}
+
+	var result ThumbnailResponse
+	result.ID = document.ID
+	result.Thumbnail = document.Thumbnail
+	result.KbID = document.KbID
+	return &result, nil
+}
+
 // ListDocumentsByDatasetID list documents by knowledge base ID
 func (s *DocumentService) ListDocumentsByDatasetID(kbID string, page, pageSize int) ([]*entity.DocumentListItem, int64, error) {
 	offset := (page - 1) * pageSize
@@ -282,7 +318,7 @@ func (s *DocumentService) ParseDocuments(datasetID, userID string, docIDs []stri
 		}
 
 		// Send task to message queue
-		
+
 	}
 
 	common.Info(fmt.Sprintf("parse documents, dataset: %s, documents: %v", datasetID, docIDs))
@@ -425,7 +461,7 @@ func (s *DocumentService) DeleteDocumentAllMetadata(docID string) error {
 
 	// Build condition to match the document
 	condition := map[string]interface{}{
-		"id":   docID,
+		"id":    docID,
 		"kb_id": doc.KbID,
 	}