diff --git a/frontend/sync-client/src/utils/is-file-type-mergable.test.ts b/frontend/sync-client/src/utils/is-file-type-mergable.test.ts
index fd316588..08972b22 100644
--- a/frontend/sync-client/src/utils/is-file-type-mergable.test.ts
+++ b/frontend/sync-client/src/utils/is-file-type-mergable.test.ts
@@ -71,3 +71,17 @@ describe("isFileTypeMergable", () => {
         );
     });
 });
+
+for (const path of [
+    "md",
+    "txt",
+    "nested/md",
+    "nested/txt",
+    "a.md/txt",
+    "a.txt/md",
+    "note."
+]) {
+    it(`does not merge an extensionless file: ${path}`, () => {
+        assert.equal(isFileTypeMergable(path, ["md", "txt"]), false);
+    });
+}
diff --git a/frontend/sync-client/src/utils/is-file-type-mergable.ts b/frontend/sync-client/src/utils/is-file-type-mergable.ts
index a895b3e2..c09deaf6 100644
--- a/frontend/sync-client/src/utils/is-file-type-mergable.ts
+++ b/frontend/sync-client/src/utils/is-file-type-mergable.ts
@@ -2,8 +2,10 @@ export function isFileTypeMergable(
     pathOrFileName: string,
     mergeableExtensions: string[]
 ): boolean {
-    const parts = pathOrFileName.split(".");
-    const fileExtension = parts.at(-1) ?? "";
+    const name = pathOrFileName.split("/").at(-1) ?? "";
+    const separator = name.lastIndexOf(".");
+    if (separator < 0) return false;
+    const fileExtension = name.slice(separator + 1);
 
     return mergeableExtensions.includes(fileExtension.toLowerCase());
 }