diff --git a/fs_attachment/models/ir_binary.py b/fs_attachment/models/ir_binary.py
index d9ea167d02..a28faa2d1b 100644
--- a/fs_attachment/models/ir_binary.py
+++ b/fs_attachment/models/ir_binary.py
@@ -70,16 +70,17 @@ def _get_stream_from(
         if stream.type == "fs":
             if mimetype:
                 stream.mimetype = mimetype
+            elif not stream.mimetype:
+                stream.mimetype = default_mimetype
             if filename:
                 stream.download_name = filename
             elif record and filename_field in record:
                 stream.download_name = record[filename_field] or stream.download_name
-
-        if (
-            not get_extension(stream.download_name)
-            and stream.mimetype != "application/octet-stream"
-        ):
-            stream.download_name += guess_extension(stream.mimetype) or ""
+            if (
+                not get_extension(stream.download_name)
+                and stream.mimetype != "application/octet-stream"
+            ):
+                stream.download_name += guess_extension(stream.mimetype) or ""
 
         return stream
 
diff --git a/fs_attachment/readme/newsfragments/684.bugfix.rst b/fs_attachment/readme/newsfragments/684.bugfix.rst
new file mode 100644
index 0000000000..554605b859
--- /dev/null
+++ b/fs_attachment/readme/newsfragments/684.bugfix.rst
@@ -0,0 +1 @@
+Fix a 500 error when downloading an attachment stored in a filesystem storage that has no mimetype: fall back to the default mimetype, as core does for attachments in the filestore.
diff --git a/fs_attachment/tests/test_stream.py b/fs_attachment/tests/test_stream.py
index 38ee7a4c0f..1a1abbe79d 100644
--- a/fs_attachment/tests/test_stream.py
+++ b/fs_attachment/tests/test_stream.py
@@ -240,3 +240,51 @@ def test_image_url_name_with_newline(self):
             },
             assert_content=self.image,
         )
+
+    def _create_attachment_without_extension(self):
+        return (
+            self.env["ir.attachment"]
+            .with_context(storage_location=self.temp_backend.code)
+            .create({"name": "report", "raw": self.content})
+        )
+
+    def test_content_url_name_without_extension(self):
+        """The extension is guessed from the mimetype for fs streams."""
+        attachment = self._create_attachment_without_extension()
+        self.assertEqual(attachment.mimetype, "text/plain")
+        self.authenticate("admin", "admin")
+        self.assertDownload(
+            f"/web/content/{attachment.id}",
+            headers={},
+            assert_status_code=200,
+            assert_headers={"Content-Disposition": "inline; filename=report.txt"},
+            assert_content=self.content,
+        )
+
+    def test_content_url_without_mimetype(self):
+        """An fs attachment without mimetype can still be downloaded.
+
+        The mimetype can be empty for attachments written outside the ORM
+        (imports, migrations). As in core, it falls back to the default
+        mimetype instead of crashing.
+        """
+        attachment = self._create_attachment_without_extension()
+        self.env.cr.execute(
+            "UPDATE ir_attachment SET mimetype = NULL WHERE id = %s", (attachment.id,)
+        )
+        attachment.invalidate_recordset(["mimetype"])
+        stream = self.env["ir.binary"]._get_stream_from(attachment)
+        self.assertEqual(stream.type, "fs")
+        self.assertEqual(stream.mimetype, "application/octet-stream")
+        self.assertEqual(stream.download_name, "report")
+        self.authenticate("admin", "admin")
+        self.assertDownload(
+            f"/web/content/{attachment.id}",
+            headers={},
+            assert_status_code=200,
+            assert_headers={
+                "Content-Type": "application/octet-stream",
+                "Content-Disposition": "inline; filename=report",
+            },
+            assert_content=self.content,
+        )