Have read_file support images

[rtfeldman]

This is very basic support for them. There are a number of other TODOs before this is really a first-class supported feature, so not adding any release notes for it; for now, this PR just makes it so that if read_file tries to read a PNG (which has come up in practice), it at least correctly sends it to Anthropic instead of messing up.

This also lays the groundwork for future PRs for more first-class support for images in tool calls across more image file formats and LLM providers.

Release Notes:

• N/A