diff --git a/Cargo.toml b/Cargo.toml
index 3c57022..0786031 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,7 +12,7 @@ edition = "2018"
 
 [dependencies]
 tokio-rustls = "0.22.0"
-tokio = { version = "1.0", features = ["fs", "io-util", "net", "rt-multi-thread"] }
+tokio = { version = "1.0", features = ["fs", "io-util", "net", "rt-multi-thread", "sync"] }
 env_logger = { version = "0.8", default-features = false, features = ["atty", "humantime", "termcolor"] }
 getopts = "0.2.21"
 log = "0.4"
diff --git a/README.md b/README.md
index fe050e7..68af2d4 100644
--- a/README.md
+++ b/README.md
@@ -50,9 +50,8 @@ The lines of the file should have this format:
 <filename>:<metadata>
 ```
 
-Where `<filename>` is just a filename (not a path) of a file in the same directory, and `<metadata>` is the metadata to be stored.
-Lines that start with optional whitespace and `#` are ignored, as are lines that do not fit the above basic format.
-Both parts are stripped of any leading and/or trailing whitespace.
+Where `<filename>` is just a filename (not a path) of a file in the same directory, and `<metadata>` is the MIME type to be stored. If `<metadata>` starts with a semicolon, agate will use the usual mechanism to determine the mime type of the file and append the specified parameters.
+Lines that start with optional whitespace and `#` are ignored, as are lines that do not contain a `:`. Both parts are stripped of any leading and/or trailing whitespace.
 
 [Gemini]: https://gemini.circumlunar.space/
 [Rust]: https://www.rust-lang.org/
diff --git a/src/main.rs b/src/main.rs
index 3f1efe1..cb9a494 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,5 @@
 mod metadata;
+use metadata::FileOptions;
 
 use {
     once_cell::sync::Lazy,
@@ -21,6 +22,7 @@ use {
         io::{AsyncReadExt, AsyncWriteExt},
         net::{TcpListener, TcpStream},
         runtime::Runtime,
+        sync::RwLock,
     },
     tokio_rustls::{server::TlsStream, TlsAcceptor},
     url::{Host, Url},
@@ -31,12 +33,19 @@ fn main() -> Result {
         env_logger::Builder::new().parse_filters("info").init();
     }
     Runtime::new()?.block_on(async {
+        let mimetypes = Arc::new(RwLock::new(FileOptions::new(
+            &ARGS
+                .language
+                .as_ref()
+                .map_or(String::new(), |lang| format!(";lang={}", lang)),
+        )));
         let listener = TcpListener::bind(&ARGS.addrs[..]).await?;
         log::info!("Listening on {:?}...", ARGS.addrs);
         loop {
             let (stream, _) = listener.accept().await?;
+            let arc = mimetypes.clone();
             tokio::spawn(async {
-                if let Err(e) = handle_request(stream).await {
+                if let Err(e) = handle_request(stream, arc).await {
                     log::error!("{:?}", e);
                 }
             });
@@ -147,11 +156,11 @@ fn check_path(s: String) -> Result<String, String> {
 }
 
 /// Handle a single client session (request + response).
-async fn handle_request(stream: TcpStream) -> Result {
+async fn handle_request(stream: TcpStream, mimetypes: Arc<RwLock<FileOptions>>) -> Result {
     let stream = &mut TLS.accept(stream).await?;
 
     match parse_request(stream).await {
-        Ok(url) => send_response(url, stream).await?,
+        Ok(url) => send_response(url, stream, mimetypes).await?,
         Err((status, msg)) => send_header(stream, status, &[msg]).await?,
     }
     stream.shutdown().await?;
@@ -223,7 +232,11 @@ async fn parse_request(
 }
 
 /// Send the client the file located at the requested URL.
-async fn send_response(url: Url, stream: &mut TlsStream<TcpStream>) -> Result {
+async fn send_response(
+    url: Url,
+    stream: &mut TlsStream<TcpStream>,
+    mimetypes: Arc<RwLock<FileOptions>>,
+) -> Result {
     let mut path = std::path::PathBuf::from(&ARGS.content_dir);
     if let Some(segments) = url.path_segments() {
         for segment in segments {
@@ -265,12 +278,21 @@ async fn send_response(url: Url, stream: &mut TlsStream<TcpStream>) -> Result {
     };
 
     // Send header.
-    if path.extension() == Some(OsStr::new("gmi")) {
-        send_text_gemini_header(stream).await?;
+    let mut locked = mimetypes.write().await;
+    let data = locked.get(&path);
+    if data.is_empty() || data.starts_with(";") {
+        // guess MIME type
+        if path.extension() == Some(OsStr::new("gmi")) {
+            send_header(stream, 20, &["text/gemini", data]).await?;
+        } else {
+            let mime = mime_guess::from_path(&path).first_or_octet_stream();
+            send_header(stream, 20, &[mime.essence_str(), data]).await?;
+        };
     } else {
-        let mime = mime_guess::from_path(&path).first_or_octet_stream();
-        send_header(stream, 20, &[mime.essence_str()]).await?;
+        // this must be a full MIME type
+        send_header(stream, 20, &[data]).await?;
     }
+    drop(locked);
 
     // Send body.
     tokio::io::copy(&mut file, stream).await?;
@@ -284,7 +306,7 @@ async fn list_directory(stream: &mut TlsStream<TcpStream>, path: &Path) -> Resul
         .add(b'?').add(b'`').add(b'{').add(b'}');
 
     log::info!("Listing directory {:?}", path);
-    send_text_gemini_header(stream).await?;
+    send_header(stream, 20, &["text/gemini"]).await?;
     let mut entries = tokio::fs::read_dir(path).await?;
     let mut lines = vec![];
     while let Some(entry) = entries.next_entry().await? {
@@ -321,11 +343,3 @@ async fn send_header(stream: &mut TlsStream<TcpStream>, status: u8, meta: &[&str
     stream.write_all(response.as_bytes()).await?;
     Ok(())
 }
-
-async fn send_text_gemini_header(stream: &mut TlsStream<TcpStream>) -> Result {
-    if let Some(lang) = ARGS.language.as_deref() {
-        send_header(stream, 20, &["text/gemini;lang=", lang]).await
-    } else {
-        send_header(stream, 20, &["text/gemini"]).await
-    }
-}