feat: add magic meta plugin implementation

Co-authored-by: aider (openai/andrew/openrouter/anthropic/claude-sonnet-4) <aider@aider.chat>

src/meta_plugin/magic.rs

@@ -0,0 +1,97 @@
+use anyhow::Result;
+use magic::{Cookie, CookieFlags};
+use std::io;
+use std::io::Write;
+
+use crate::meta_plugin::MetaPlugin;
+
+#[derive(Debug, Clone)]
+pub struct MagicMetaPlugin {
+    buffer: Vec<u8>,
+    meta_name: String,
+    cookie_flags: CookieFlags,
+}
+
+impl MagicMetaPlugin {
+    pub fn new_file_type() -> MagicMetaPlugin {
+        MagicMetaPlugin {
+            buffer: Vec::new(),
+            meta_name: "magic_file_type".to_string(),
+            cookie_flags: CookieFlags::NONE,
+        }
+    }
+
+    pub fn new_mime_type() -> MagicMetaPlugin {
+        MagicMetaPlugin {
+            buffer: Vec::new(),
+            meta_name: "magic_mime_type".to_string(),
+            cookie_flags: CookieFlags::MIME_TYPE,
+        }
+    }
+
+    pub fn new_mime_encoding() -> MagicMetaPlugin {
+        MagicMetaPlugin {
+            buffer: Vec::new(),
+            meta_name: "magic_mime_encoding".to_string(),
+            cookie_flags: CookieFlags::MIME_ENCODING,
+        }
+    }
+}
+
+impl MetaPlugin for MagicMetaPlugin {
+    fn is_internal(&self) -> bool {
+        true
+    }
+    
+    fn create(&self) -> Result<Box<dyn Write>> {
+        // For meta plugins, we don't actually create a writer since we're buffering data internally
+        Ok(Box::new(DummyWriter))
+    }
+
+    fn finalize(&mut self) -> io::Result<String> {
+        let cookie = Cookie::open(self.cookie_flags)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, format!("Failed to open magic cookie: {}", e)))?;
+
+        cookie.load(&Default::default())
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, format!("Failed to load magic database: {}", e)))?;
+
+        let result = cookie.buffer(&self.buffer)
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, format!("Failed to analyze buffer: {}", e)))?;
+
+        // Clean up the result - remove extra whitespace and take first part if needed
+        let trimmed = result.trim();
+        
+        // For some magic results, we might want just the first part before semicolon or comma
+        let cleaned = if trimmed.contains(';') {
+            trimmed.split(';').next().unwrap_or(trimmed).trim()
+        } else if trimmed.contains(',') && self.meta_name.contains("mime") {
+            trimmed.split(',').next().unwrap_or(trimmed).trim()
+        } else {
+            trimmed
+        };
+
+        Ok(cleaned.to_string())
+    }
+
+    fn update(&mut self, data: &[u8]) {
+        self.buffer.extend_from_slice(data);
+    }
+
+    fn meta_name(&mut self) -> String {
+        self.meta_name.clone()
+    }
+}
+
+// Dummy writer that implements Write but doesn't do anything
+// This is needed to satisfy the MetaPlugin trait requirements
+struct DummyWriter;
+
+impl Write for DummyWriter {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}