diff --git a/internal/router/static_router.go b/internal/router/static_router.go
index e16002cfc..290148b62 100644
--- a/internal/router/static_router.go
+++ b/internal/router/static_router.go
@@ -68,7 +68,8 @@ func (a *StaticRouter) RegisterStaticRouter(r *gin.RouterGroup) {
 }
 
 func attachmentFileLocalPath(uploadPath, requestPath, originalFilename string) (string, bool) {
-	realFilename := strings.TrimSuffix(requestPath, "/"+originalFilename) + filepath.Ext(originalFilename)
+	// The uploader saves attachments with a lowercased extension, so match it here.
+	realFilename := strings.TrimSuffix(requestPath, "/"+originalFilename) + strings.ToLower(filepath.Ext(originalFilename))
 	attachmentRoot := filepath.Join(uploadPath, constant.FilesPostSubPath)
 	fileLocalPath := filepath.Join(attachmentRoot, realFilename)
 	relPath, err := filepath.Rel(attachmentRoot, fileLocalPath)
diff --git a/internal/router/static_router_test.go b/internal/router/static_router_test.go
index b123c95db..21a83e0ad 100644
--- a/internal/router/static_router_test.go
+++ b/internal/router/static_router_test.go
@@ -20,10 +20,15 @@
 package router
 
 import (
+	"net/http"
+	"net/http/httptest"
+	"os"
 	"path/filepath"
 	"testing"
 
 	"github.com/apache/answer/internal/base/constant"
+	"github.com/apache/answer/internal/service/service_config"
+	"github.com/gin-gonic/gin"
 )
 
 func TestAttachmentFileLocalPathRejectsTraversal(t *testing.T) {
@@ -44,3 +49,43 @@ func TestAttachmentFileLocalPathRejectsTraversal(t *testing.T) {
 		}
 	}
 }
+
+// The uploader stores attachments under a lowercased extension (Report.PDF is
+// saved as hash.pdf), so the download route must look the file up the same way.
+func TestAttachmentFileLocalPathLowercasesExtension(t *testing.T) {
+	uploadPath := t.TempDir()
+
+	filePath, ok := attachmentFileLocalPath(uploadPath, "/hash/Report.PDF", "Report.PDF")
+	if !ok {
+		t.Fatal("valid attachment path was rejected")
+	}
+	want := filepath.Join(uploadPath, constant.FilesPostSubPath, "hash.pdf")
+	if filePath != want {
+		t.Fatalf("attachment path = %q, want %q", filePath, want)
+	}
+}
+
+func TestAttachmentDownloadWithUppercaseExtension(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	uploadPath := t.TempDir()
+	attachmentDir := filepath.Join(uploadPath, constant.FilesPostSubPath)
+	if err := os.MkdirAll(attachmentDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	// This is the name UploadPostAttachment gives an upload called Report.PDF.
+	if err := os.WriteFile(filepath.Join(attachmentDir, "hash.pdf"), []byte("%PDF-1.4"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	r := gin.New()
+	NewStaticRouter(&service_config.ServiceConfig{UploadPath: uploadPath}).RegisterStaticRouter(&r.RouterGroup)
+
+	w := httptest.NewRecorder()
+	r.ServeHTTP(w, httptest.NewRequest(http.MethodGet, "/uploads/files/post/hash/Report.PDF", nil))
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d (location %q), want %d", w.Code, w.Header().Get("Location"), http.StatusOK)
+	}
+	if got := w.Body.String(); got != "%PDF-1.4" {
+		t.Fatalf("body = %q, want the stored attachment", got)
+	}
+}