@prem_k more docs here: https://llm.datasette.io/en/stable/plugins/advanced-model-plugins.html#attachments-for-multi-modal-models

Implementations are spread out across different plugins, eg https://github.com/simonw/llm/blob/a44ba49c21f8d4ac30c8e41bfa5599c258ce53cc/llm/default_plugins/openai_models.py#L338 and https://github.com/simonw/llm-gemini/blob/ce82727a6950c7769a8e40bf030591d0e6f83e5e/llm_gemini.py#L135