diff --git a/crates/kebab-app/src/external.rs b/crates/kebab-app/src/external.rs
index 61fe8a6..7a51990 100644
--- a/crates/kebab-app/src/external.rs
+++ b/crates/kebab-app/src/external.rs
@@ -78,12 +78,17 @@ pub fn copy_to_external(
 /// markdown string. Errors if `body` already starts with `---` (the user
 /// should use `ingest_file_with_config` for files that already carry
 /// frontmatter).
+///
+/// Internal `yaml_quote` always uses double-quoted YAML form with backslash
+/// escapes for `"` / `\` / control chars — agent-supplied titles with
+/// special characters are safe.
 pub fn inject_frontmatter(
     body: &str,
     title: &str,
     source_uri: Option<&str>,
 ) -> Result<String> {
-    if body.trim_start().starts_with("---\n") || body.trim_start().starts_with("---\r\n") {
+    let head = body.trim_start();
+    if head.starts_with("---\n") || head.starts_with("---\r\n") || head.starts_with("---\r") {
         anyhow::bail!(
             "stdin already has frontmatter; use `kebab ingest-file` for files with metadata"
         );
diff --git a/crates/kebab-app/src/lib.rs b/crates/kebab-app/src/lib.rs
index 20ee48b..fa5b242 100644
--- a/crates/kebab-app/src/lib.rs
+++ b/crates/kebab-app/src/lib.rs
@@ -1899,10 +1899,19 @@ pub fn ingest_file_with_config(
         anyhow::bail!("ingest-file: not a regular file: {}", path.display());
     }
 
-    let ext = path
+    let ext_raw = path
         .extension()
         .and_then(|e| e.to_str())
         .ok_or_else(|| anyhow::anyhow!("ingest-file: source has no extension: {}", path.display()))?;
+    let ext = ext_raw.to_lowercase();
+
+    const SUPPORTED_EXTS: &[&str] = &["md", "pdf", "png", "jpg", "jpeg"];
+    if !SUPPORTED_EXTS.contains(&ext.as_str()) {
+        anyhow::bail!(
+            "ingest-file: unsupported extension `.{}` (supported: {:?})",
+            ext, SUPPORTED_EXTS
+        );
+    }
 
     let bytes = std::fs::read(path)
         .with_context(|| format!("ingest-file: read source {}", path.display()))?;
@@ -1925,7 +1934,7 @@ pub fn ingest_file_with_config(
         .context("ingest-file: append _external/ to .kebabignore")?;
 
     // Copy bytes to _external/<hash>.<ext>.
-    let dest = crate::external::copy_to_external(&external_dir, &bytes, ext)
+    let dest = crate::external::copy_to_external(&external_dir, &bytes, &ext)
         .context("ingest-file: copy to _external")?;
 
     // Build a SourceScope that targets _external/ with include filter
@@ -1963,6 +1972,11 @@ pub fn ingest_stdin_with_config(
     let wrapped = crate::external::inject_frontmatter(body, title, source_uri)?;
 
     let workspace_root = config.resolve_workspace_root();
+    // Note: ensure_external_dir + ensure_kebabignore_entry + copy_to_external
+    // are called here AND inside ingest_file_with_config. All three are
+    // idempotent; the redundancy is intentional — keeping stdin's wrapped
+    // bytes accessible by `ingest_file_with_config` requires the dest path
+    // to exist. The ~ms double-stat overhead is negligible at v1 scale.
     let external_dir = crate::external::ensure_external_dir(&workspace_root)?;
     crate::external::ensure_kebabignore_entry(&workspace_root)?;
 
diff --git a/crates/kebab-app/tests/ingest_file.rs b/crates/kebab-app/tests/ingest_file.rs
index b70fa4d..85255f6 100644
--- a/crates/kebab-app/tests/ingest_file.rs
+++ b/crates/kebab-app/tests/ingest_file.rs
@@ -87,3 +87,25 @@ fn ingest_file_errors_on_missing_path() {
     let err = kebab_app::ingest_file_with_config(cfg, &nonexistent).unwrap_err();
     assert!(err.to_string().contains("does not exist"), "{err}");
 }
+
+#[test]
+fn ingest_file_errors_on_unsupported_extension() {
+    let dir = tempfile::tempdir().unwrap();
+    let workspace = dir.path().join("notes");
+    let data = dir.path().join("data");
+    fs::create_dir_all(&workspace).unwrap();
+    fs::create_dir_all(&data).unwrap();
+
+    let mut cfg = Config::defaults();
+    cfg.workspace.root = workspace.to_string_lossy().into_owned();
+    cfg.storage.data_dir = data.to_string_lossy().into_owned();
+    cfg.models.embedding.provider = "none".to_string();
+    cfg.models.embedding.dimensions = 0;
+
+    let docx = dir.path().join("doc.docx");
+    fs::write(&docx, b"fake docx bytes").unwrap();
+
+    let err = kebab_app::ingest_file_with_config(cfg, &docx).unwrap_err();
+    assert!(err.to_string().contains("unsupported extension"), "{err}");
+    assert!(err.to_string().contains(".docx") || err.to_string().contains("docx"), "{err}");
+}