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