#10721: fix for matrix media: destructure downloadContent return value in media handler

`MatrixClient.downloadContent()` returns `{ data: Buffer, contentType: string }`, not a raw Buffer. The old code called `Buffer.from()` on the wrapper object, which threw a `TypeError` on every media download. The error was silently caught, leaving media as null — so the bot only received the text fallback body (e.g. "Voice message") instead of the actual media file. Tested in Element X with voice messages on a live Matrix instance. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Fixes Matrix media downloads by correctly destructuring `MatrixClient.downloadContent()` to get the `data` `Buffer` instead of treating the wrapper object as a `Buffer`. - Enforces the configured max size check against the downloaded buffer, preventing oversized media from being saved. - Preserves/propagates the server-provided content type (`contentType` → `headerType`) into the media save path so downstream handling can infer MIME type correctly. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is small, localized, and directly fixes a concrete runtime error path by aligning with the actual `downloadContent()` return shape; it preserves existing control flow and error handling while improving MIME type propagation. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->