diff --git a/src/dnd.rs b/src/dnd.rs
new file mode 100644
index 0000000..f0d2630
--- /dev/null
+++ b/src/dnd.rs
@@ -0,0 +1,152 @@
+//! Drag-and-drop out of a preview thumbnail: what is offered, how it is
+//! written, and what the thumbnail does afterwards. No Wayland in here.
+
+use std::io::Write;
+use std::path::Path;
+
+use anyhow::{Context, Result, bail};
+
+/// What a drag offers; the receiving app picks one. File managers and chat
+/// apps take the file, image editors take the pixels.
+pub const MIME_TYPES: [&str; 2] = ["text/uri-list", "image/png"];
+
+/// `file://` URI for an absolute path, with everything but RFC 3986
+/// unreserved characters and `/` percent-encoded byte by byte. The default
+/// file names have spaces; Turkish names have non-ASCII.
+pub fn file_uri(path: &Path) -> String {
+    let mut uri = String::from("file://");
+    for &b in path.as_os_str().as_encoded_bytes() {
+        if b.is_ascii_alphanumeric() || b"-._~/".contains(&b) {
+            uri.push(b as char);
+        } else {
+            uri.push_str(&format!("%{b:02X}"));
+        }
+    }
+    uri
+}
+
+/// Writes the screenshot at `path` in the requested `mime` type.
+pub fn write_offer(mime: &str, path: &Path, out: &mut impl Write) -> Result<()> {
+    match mime {
+        "text/uri-list" => out.write_all(format!("{}\r\n", file_uri(path)).as_bytes())?,
+        "image/png" => {
+            let bytes = std::fs::read(path)
+                .with_context(|| format!("could not read {}", path.display()))?;
+            out.write_all(&bytes)?;
+        }
+        other => bail!("nobody offered {other}"),
+    }
+    out.flush()?;
+    Ok(())
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Outcome {
+    /// Dropped on an app that took it.
+    Dropped,
+    /// Released on nothing, rejected, or Esc.
+    Cancelled,
+}
+
+/// What the thumbnail does once its drag is over.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum AfterDrag {
+    Close,
+    ComeBack,
+    SlideOut,
+}
+
+/// `expired`: the thumbnail's timer fired while it was being dragged.
+pub fn after_drag(outcome: Outcome, expired: bool) -> AfterDrag {
+    match (outcome, expired) {
+        (Outcome::Dropped, _) => AfterDrag::Close,
+        (Outcome::Cancelled, false) => AfterDrag::ComeBack,
+        (Outcome::Cancelled, true) => AfterDrag::SlideOut,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Read;
+
+    #[test]
+    fn plain_path() {
+        assert_eq!(file_uri(Path::new("/tmp/a.png")), "file:///tmp/a.png");
+    }
+
+    #[test]
+    fn spaces_are_encoded() {
+        assert_eq!(
+            file_uri(Path::new("/home/u/Screenshot 2026-10-01 at 14.03.22.png")),
+            "file:///home/u/Screenshot%202026-10-01%20at%2014.03.22.png"
+        );
+    }
+
+    #[test]
+    fn non_ascii_is_encoded_as_utf8() {
+        assert_eq!(
+            file_uri(Path::new("/tmp/ışık.png")),
+            "file:///tmp/%C4%B1%C5%9F%C4%B1k.png"
+        );
+    }
+
+    #[test]
+    fn percent_and_hash_are_encoded() {
+        assert_eq!(
+            file_uri(Path::new("/tmp/50%#1.png")),
+            "file:///tmp/50%25%231.png"
+        );
+    }
+
+    /// Writes through a real pipe, as a receiving app would read it.
+    fn through_pipe(mime: &str, path: &Path) -> Result<Vec<u8>> {
+        let (reader, writer) = rustix::pipe::pipe()?;
+        let mut writer = std::fs::File::from(writer);
+        let path = path.to_path_buf();
+        let mime = mime.to_string();
+        let handle = std::thread::spawn(move || write_offer(&mime, &path, &mut writer));
+        let mut out = Vec::new();
+        std::fs::File::from(reader).read_to_end(&mut out)?;
+        handle.join().unwrap()?;
+        Ok(out)
+    }
+
+    #[test]
+    fn uri_list_is_one_crlf_line() {
+        let out = through_pipe("text/uri-list", Path::new("/tmp/a b.png")).unwrap();
+        assert_eq!(out, b"file:///tmp/a%20b.png\r\n");
+    }
+
+    #[test]
+    fn png_is_the_file_bytes() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("shot.png");
+        // Larger than a pipe buffer, so the writer really has to wait.
+        let bytes: Vec<u8> = (0..200_000u32).map(|i| i as u8).collect();
+        std::fs::write(&path, &bytes).unwrap();
+        assert_eq!(through_pipe("image/png", &path).unwrap(), bytes);
+    }
+
+    #[test]
+    fn missing_file_is_an_error_not_a_panic() {
+        let err = through_pipe("image/png", Path::new("/nonexistent/x.png")).unwrap_err();
+        assert!(
+            format!("{err:#}").contains("could not read /nonexistent/x.png"),
+            "{err:#}"
+        );
+    }
+
+    #[test]
+    fn unknown_mime_is_an_error() {
+        assert!(through_pipe("text/html", Path::new("/tmp/a.png")).is_err());
+    }
+
+    #[test]
+    fn after_drag_decisions() {
+        assert_eq!(after_drag(Outcome::Dropped, false), AfterDrag::Close);
+        assert_eq!(after_drag(Outcome::Dropped, true), AfterDrag::Close);
+        assert_eq!(after_drag(Outcome::Cancelled, false), AfterDrag::ComeBack);
+        assert_eq!(after_drag(Outcome::Cancelled, true), AfterDrag::SlideOut);
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index cf00df4..59fa3ba 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,6 +4,7 @@
 mod capture;
 mod config;
 mod detach;
+mod dnd;
 mod doctor;
 mod error;
 mod frame;