diff --git a/crates/engine/src/cmd/swatchlib.rs b/crates/engine/src/cmd/swatchlib.rs
index d3652aa4e..50033eff7 100644
--- a/crates/engine/src/cmd/swatchlib.rs
+++ b/crates/engine/src/cmd/swatchlib.rs
@@ -332,14 +332,17 @@ fn library_tab_name(file: &str, lib_name: &str) -> String {
 
 /// The file name of library `name` with extension `ext`: characters file systems reject become
 /// `-`, and dots and spaces at either end are dropped. A name with nothing left is `Library`, and
-/// one that is a Windows device name (`CON`, `NUL`, `COM1`…) gets a `-` after it.
+/// one that is a Windows device name (`CON`, `NUL`, `COM1`…, also with an extension) gets a `-`
+/// after its first part (`CON-.json`, `CON-.txt.json`).
 pub fn library_file_name(name: &str, ext: &str) -> String {
     let base: String = name.chars().map(|c| if "/\\:*?\"<>|".contains(c) || c.is_control() { '-' } else { c }).collect();
     let base = base.trim_matches(['.', ' ']);
     if base.is_empty() {
         format!("Library.{ext}")
     } else if crate::file_access::is_device_name(base) {
-        format!("{base}-.{ext}")
+        // The part before the first dot is what makes it a device, so the `-` goes there.
+        let (stem, rest) = base.split_once('.').map_or((base, String::new()), |(stem, rest)| (stem, format!(".{rest}")));
+        format!("{stem}-{rest}.{ext}")
     } else {
         format!("{base}.{ext}")
     }
diff --git a/crates/engine/src/tests/swatchlib.rs b/crates/engine/src/tests/swatchlib.rs
index d6c9f8452..7337d6b74 100644
--- a/crates/engine/src/tests/swatchlib.rs
+++ b/crates/engine/src/tests/swatchlib.rs
@@ -642,11 +642,14 @@ fn gradient_libraries_are_listed_and_gradient_swatches_rename_and_take_new_gradi
 #[test]
 fn a_library_file_name_is_never_a_windows_device_name() {
     use crate::cmd::swatchlib::library_file_name;
-    for (title, ext) in [("CON", "json"), ("nul", "json"), ("Com1", "acb"), ("LPT9", "ase"), ("aux.", "json")] {
+    for (title, ext) in
+        [("CON", "json"), ("nul", "json"), ("Com1", "acb"), ("LPT9", "ase"), ("aux.", "json"), ("CON.txt", "json"), ("nul.tar.gz", "acb")]
+    {
         let file = library_file_name(title, ext);
         assert!(!crate::file_access::is_device_name(&file), "{title}: {file}");
         assert!(
-            file.ends_with(&format!(".{ext}")) && file.to_ascii_lowercase().contains(&title.trim_end_matches('.').to_ascii_lowercase()),
+            file.ends_with(&format!(".{ext}"))
+                && file.to_ascii_lowercase().starts_with(&title.split('.').next().unwrap_or_default().to_ascii_lowercase()),
             "{title}: {file}"
         );
     }