diff --git a/crates/zx_document_core/Cargo.toml b/crates/zx_document_core/Cargo.toml
index 48372f4..70bb2a4 100644
--- a/crates/zx_document_core/Cargo.toml
+++ b/crates/zx_document_core/Cargo.toml
@@ -6,3 +6,5 @@ edition = "2021"
 [dependencies]
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+infer = "0.16"
+mime_guess = "2"
diff --git a/crates/zx_document_core/src/material_type.rs b/crates/zx_document_core/src/material_type.rs
index 85ca0a1..926ff15 100644
--- a/crates/zx_document_core/src/material_type.rs
+++ b/crates/zx_document_core/src/material_type.rs
@@ -1,5 +1,9 @@
+use std::path::Path;
+
 use serde::{Deserialize, Serialize};
 
+use crate::error::DocumentError;
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub enum MaterialType {
     Markdown,
@@ -20,3 +24,188 @@ pub enum PreviewMode {
     ExternalOpen,
     Unsupported,
 }
+
+impl MaterialType {
+    pub fn preview_mode(&self) -> PreviewMode {
+        match self {
+            MaterialType::Markdown | MaterialType::Text | MaterialType::Image => {
+                PreviewMode::NativeReader
+            }
+            MaterialType::Pdf | MaterialType::Word | MaterialType::Excel => {
+                PreviewMode::PlatformPreview
+            }
+            MaterialType::PowerPoint => PreviewMode::ExternalOpen,
+            MaterialType::Epub => PreviewMode::NativeReader,
+            MaterialType::Unknown => PreviewMode::Unsupported,
+        }
+    }
+}
+
+/// Detect MaterialType from a file path.
+///
+/// Strategy: magic bytes → MIME → extension
+pub fn detect_material_type(file_path: &str) -> Result<MaterialType, DocumentError> {
+    let path = Path::new(file_path);
+
+    // 1. Read file header for magic bytes detection
+    if let Ok(buf) = std::fs::read(file_path) {
+        if let Some(info) = infer::get(&buf) {
+            let inferred = match info.mime_type() {
+                "application/pdf" => Some(MaterialType::Pdf),
+                "image/png" | "image/jpeg" | "image/webp" | "image/gif" => Some(MaterialType::Image),
+                "application/epub+zip" => Some(MaterialType::Epub),
+                "application/vnd.openxmlformats-officedocument.wordprocessingml.document"
+                | "application/msword" => Some(MaterialType::Word),
+                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
+                | "application/vnd.ms-excel" => Some(MaterialType::Excel),
+                "application/vnd.openxmlformats-officedocument.presentationml.presentation"
+                | "application/vnd.ms-powerpoint" => Some(MaterialType::PowerPoint),
+                _ => None,
+            };
+            if let Some(mt) = inferred {
+                return Ok(mt);
+            }
+        }
+    }
+
+    // 2. Try MIME guess from extension
+    if let Some(ext) = path.extension().and_then(|e| e.to_str()) {
+        let mime = mime_guess::from_ext(ext).first_or_octet_stream();
+        match mime.type_() {
+            mime_guess::mime::TEXT => {
+                if mime.subtype().as_str() == "markdown"
+                    || mime.subtype().as_str() == "x-markdown"
+                {
+                    return Ok(MaterialType::Markdown);
+                }
+                return match ext {
+                    "md" | "markdown" => Ok(MaterialType::Markdown),
+                    "txt" | "text" => Ok(MaterialType::Text),
+                    "html" | "htm" | "css" | "js" | "ts" | "rs" | "py" | "java"
+                    | "c" | "cpp" | "h" | "hpp" | "swift" | "kt" | "xml" | "json"
+                    | "yaml" | "yml" | "toml" => Ok(MaterialType::Text),
+                    _ => Ok(MaterialType::Unknown),
+                };
+            }
+            mime_guess::mime::APPLICATION => {
+                return match ext {
+                    "pdf" => Ok(MaterialType::Pdf),
+                    "epub" => Ok(MaterialType::Epub),
+                    "doc" | "docx" => Ok(MaterialType::Word),
+                    "xls" | "xlsx" => Ok(MaterialType::Excel),
+                    "ppt" | "pptx" => Ok(MaterialType::PowerPoint),
+                    _ => Ok(MaterialType::Unknown),
+                };
+            }
+            mime_guess::mime::IMAGE => return Ok(MaterialType::Image),
+            _ => {}
+        }
+    }
+
+    // 3. Extension fallback
+    if let Some(ext) = path.extension().and_then(|e| e.to_str()) {
+        return Ok(match ext {
+            "md" | "markdown" => MaterialType::Markdown,
+            "txt" | "text" => MaterialType::Text,
+            "pdf" => MaterialType::Pdf,
+            "png" | "jpg" | "jpeg" | "webp" | "gif" | "bmp" | "svg" => MaterialType::Image,
+            "epub" => MaterialType::Epub,
+            "doc" | "docx" => MaterialType::Word,
+            "xls" | "xlsx" => MaterialType::Excel,
+            "ppt" | "pptx" => MaterialType::PowerPoint,
+            _ => MaterialType::Unknown,
+        });
+    }
+
+    Ok(MaterialType::Unknown)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_detect_markdown() {
+        let t = detect_material_type("test.md").unwrap();
+        assert_eq!(t, MaterialType::Markdown);
+    }
+
+    #[test]
+    fn test_detect_txt() {
+        let t = detect_material_type("notes.txt").unwrap();
+        assert_eq!(t, MaterialType::Text);
+    }
+
+    #[test]
+    fn test_detect_pdf() {
+        let t = detect_material_type("doc.pdf").unwrap();
+        assert_eq!(t, MaterialType::Pdf);
+    }
+
+    #[test]
+    fn test_detect_image() {
+        assert_eq!(
+            detect_material_type("photo.png").unwrap(),
+            MaterialType::Image
+        );
+        assert_eq!(
+            detect_material_type("photo.jpg").unwrap(),
+            MaterialType::Image
+        );
+        assert_eq!(
+            detect_material_type("photo.jpeg").unwrap(),
+            MaterialType::Image
+        );
+    }
+
+    #[test]
+    fn test_detect_epub() {
+        let t = detect_material_type("book.epub").unwrap();
+        assert_eq!(t, MaterialType::Epub);
+    }
+
+    #[test]
+    fn test_detect_office() {
+        assert_eq!(
+            detect_material_type("report.docx").unwrap(),
+            MaterialType::Word
+        );
+        assert_eq!(
+            detect_material_type("sheet.xlsx").unwrap(),
+            MaterialType::Excel
+        );
+        assert_eq!(
+            detect_material_type("deck.pptx").unwrap(),
+            MaterialType::PowerPoint
+        );
+    }
+
+    #[test]
+    fn test_detect_unknown() {
+        let t = detect_material_type("weird.xyz").unwrap();
+        assert_eq!(t, MaterialType::Unknown);
+    }
+
+    #[test]
+    fn test_detect_no_extension() {
+        let t = detect_material_type("README").unwrap();
+        assert_eq!(t, MaterialType::Unknown);
+    }
+
+    #[test]
+    fn test_preview_mode() {
+        assert_eq!(
+            MaterialType::Markdown.preview_mode(),
+            PreviewMode::NativeReader
+        );
+        assert_eq!(MaterialType::Pdf.preview_mode(), PreviewMode::PlatformPreview);
+        assert_eq!(
+            MaterialType::PowerPoint.preview_mode(),
+            PreviewMode::ExternalOpen
+        );
+        assert_eq!(
+            MaterialType::Unknown.preview_mode(),
+            PreviewMode::Unsupported
+        );
+    }
+}