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.lock

@@ -161,6 +161,7 @@ dependencies = [
  "affine_common",
  "chrono",
  "file-format",
+ "infer",
  "mimalloc",
  "napi",
  "napi-build",
@@ -1504,9 +1505,9 @@ dependencies = [
 
 [[package]]
 name = "file-format"
-version = "0.26.0"
+version = "0.28.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e7ef3d5e8ae27277c8285ac43ed153158178ef0f79567f32024ca8140a0c7cd8"
+checksum = "0eab8aa2fba5f39f494000a22f44bf3c755b7d7f8ffad3f36c6d507893074159"
 
 [[package]]
 name = "flate2"
@@ -1913,7 +1914,7 @@ dependencies = [
  "js-sys",
  "log",
  "wasm-bindgen",
- "windows-core 0.57.0",
+ "windows-core 0.61.2",
 ]
 
 [[package]]