feat: improve attachment headers (#13709)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Safer, consistent file downloads with automatic attachment headers and
filenames.
- Smarter MIME detection for uploads (avatars, workspace blobs, Copilot
files/transcripts).
  - Sensible default buffer limit when reading uploads.

- **Bug Fixes**
- Prevents risky content from rendering inline by forcing downloads and
adding no‑sniff protection.
- More accurate content types when original metadata is missing or
incorrect.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Cargo.toml

@@ -39,7 +39,7 @@ crossbeam-channel      = "0.5"
 dispatch2              = "0.3"
 docx-parser            = { git = "https://github.com/toeverything/docx-parser" }
 dotenvy                = "0.15"
-file-format            = { version = "0.26", features = ["reader"] }
+file-format            = { version = "0.28", features = ["reader"] }
 homedir                = "0.3"
 infer                  = { version = "0.19.0" }
 lasso                  = { version = "0.7", features = ["multi-threaded"] }