Be advised there may or may not be anything deserving of a warning in the summary field. It's purely a Mastodon convention.

The de facto usage definition comes from the early work on RSS by news organisations in the mid 90s. What they wanted was

title
summary (brief description)
body

Most syndication/federation protocols were built around that model. ActivityStreams maps these to name, summary, and content. Content warnings really belong elsewhere, but this wasn't offered in the AS spec. This might be a good subject for an FEP, so that there's something well-defined to use instead of summary. I for one would adopt it in a heartbeat to get our summary field back.  

My advice to @Gregory would be to ignore summary completely on DMs if they don't fit your UX design, but hide images if sensitive is set. People can lose their jobs (or got to jail) over "indecent" images popping up without warning.