Saving an RGBA image directly as JPEG will cause an error. If the image
is in RGBA mode, convert it to RGB mode before saving it in JPG format.

### What problem does this PR solve?

During document parsing in the knowledge base, we occasionally encounter
the error 'cannot write mode RGBA as JPEG.' This occurs because images
in RGBA mode cannot be directly saved as JPEG. They must be converted
first before saving.

### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)