Handle attachments that are sent without a prompt

[simonw]

llm-gemini breaks on this:

llm -m gemini-1.5-flash-latest -a joke-about-pelicans-and-walruses.png

Error: * GenerateContentRequest.contents[0].parts[0].data: required oneof field 'data' must have one initialized field

Originally posted by @simonw in simonw/llm#611 (comment)

[simonw]

Interesting - Gemini transcribes the text from an image without following the instructions:

llm -m gemini-1.5-flash-latest -a joke-about-pelicans-and-walruses.png

Joke about pelicans and walruses living together.

But it follows instructions in an audio file:

llm -m gemini-1.5-flash-latest -a https://static.simonwillison.net/static/2024/pelican-joke-request.mp3

Why did the pelican cross the road?

To get to the other tide!

[simonw]

Can't release this until I release the underlying LLM update, since without that it's not possible to send an attachment without a prompt - at least using the CLI.