unexpected error when given an unknown mimetype

[0x1eef]

Problem

We don't have a mime type for markdown documents (eg files with the extension .md)
and we don't implement a fallback mime type either. This leads to a confusing error deep
within LLM::Multipart. Reproduction steps:

#!/usr/bin/env ruby

llm = LLM.openai(key: ENV["OPENAI_SECRET"])
llm.files.create(file: "/path/to/file.md") # boom

Solutions

• Fallback on the mime type application/octet-stream for unknown file extensions
• Add a mime type of text/markdown for markdown documents (md, markdown, mkd).