图片上传可以用Base64

**Is your feature request related to a problem? Please describe.**

上传本地图片显示Request failed: NETERROR.
使用URL倒是可以，但是和本地部署（很多是因为资源不能上网才本地部署的，否则就直接用正式版的coze了）场景冲突

**Describe the solution you'd like**

上传图片的时候，直接（超尺寸的话先压缩一下）后转Base64一起传给大模型
或者提供一个选项，我看代码里好像有这部分，不知道为何没有生效

Trace the local image upload path and inspect the existing Base64-related code or option mentioned in the issue. Confirm how uploaded images are sent to the model and define completion as local images working without external URLs, including behavior for oversized images. No specific files or tests are identified, so repository-wide code reading will be needed.

Written by the indexing model from the issue text.