Fix file upload bug on server

Author: hazelnutcloud

src/treaty2/index.ts

@@ -289,8 +289,13 @@ const createProxy = (
 							fetchInit.body
 						)) {
 							if (isServer) {
-								formData.append(key, field as any)
-
+                if (Array.isArray(field)) {
+                  for (const f of field) {
+                    formData.append(key, f)
+                  }
+                } else {
+                  formData.append(key, field as any)
+                }
 								continue
 							}