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 -->

packages/backend/native/Cargo.toml

@@ -10,6 +10,7 @@ crate-type = ["cdylib"]
 affine_common = { workspace = true, features = ["doc-loader"] }
 chrono        = { workspace = true }
 file-format   = { workspace = true }
+infer         = { workspace = true }
 napi          = { workspace = true, features = ["async"] }
 napi-derive   = { workspace = true }
 rand          = { workspace = true }

packages/backend/native/src/file_type.rs

@@ -2,7 +2,11 @@ use napi_derive::napi;
 
 #[napi]
 pub fn get_mime(input: &[u8]) -> String {
-  file_format::FileFormat::from_bytes(input)
-    .media_type()
-    .to_string()
+  if let Some(kind) = infer::get(&input[..4096.min(input.len())]) {
+    kind.mime_type().to_string()
+  } else {
+    file_format::FileFormat::from_bytes(input)
+      .media_type()
+      .to_string()
+  }
 }