diff --git a/CHANGELOG.md b/CHANGELOG.md
index de09a109c..e51ea1f31 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -38,6 +38,7 @@ version, see the upgrade guides in `docs/`.
 - **router:** The text after `**` is matched literally, e.g. `/f/**/x.json` doesn't match `/f/a/xajson` anymore, and params after `**` can have any name, e.g. `:my-id`, which threw an `Invalid capture group name` error (#333).
 - **router:** Params starting with `$` keep their names next to wildcards, e.g. `$x` in `/a/*/:$x`, instead of being renamed to `$` or `$$` and replacing the wildcard's value (#334).
 - **admin:** Everything behind the topmost dialog is inert: the page, the navigation, the sidebar and other dialogs. Select All in a dialog no longer selects the values behind it, and assistive technology can't reach them either.
+- **server:** `toJSON()` signs copies of the asset files inside JSON columns, e.g. `content.header.file`, where it replaced the record's own file objects with the signed copies (#388).
 - **utils:** `isUrl()` returns `false` for invalid URLs with long paths right away, where the time doubled with each path segment, taking seconds for 28 segments. URLs ending in a lone `?` or `#` are valid too (#348).
 - **utils:** `clone()` copies typed arrays, and Node.js buffers stay buffers, where it copied their elements one by one into a plain object (#350).
 - **utils:** `deindent` interpolates `null` and `undefined` as in template literals, where it threw a `TypeError` (#352).
diff --git a/packages/server/src/models/Model.js b/packages/server/src/models/Model.js
index 5c81648bd..0ac8485e7 100644
--- a/packages/server/src/models/Model.js
+++ b/packages/server/src/models/Model.js
@@ -13,7 +13,6 @@ import {
   parseDataPath,
   normalizeDataPath,
   getEntriesAtDataPath,
-  setValueAtDataPath,
   groupBy,
   assignDeeply,
   formatPlainDate,
@@ -1108,10 +1107,23 @@ export class Model extends objection.Model {
   static _mapAssetFiles(json, callback) {
     const { assets } = this.definition
     if (assets) {
-      // Write each file back to its own location:
+      // Write each file back to its own location, into copies of the objects
+      // along its path, as nested objects are shared with the model:
+      const copies = new Set()
       for (const { file, path, dataPath } of this._getAssetFiles(json)) {
         const storage = this.app.getStorage(assets[dataPath].storage)
-        setValueAtDataPath(json, path, callback(file, storage))
+        const parts = parseDataPath(path)
+        const last = parts.pop()
+        let parent = json
+        for (const part of parts) {
+          let value = parent[part]
+          if (!copies.has(value)) {
+            value = parent[part] = isArray(value) ? [...value] : { ...value }
+            copies.add(value)
+          }
+          parent = value
+        }
+        parent[last] = callback(file, storage)
       }
     }
   }
diff --git a/tests/server/assets.test.ts b/tests/server/assets.test.ts
index 2e8d27c5e..920d3fc07 100644
--- a/tests/server/assets.test.ts
+++ b/tests/server/assets.test.ts
@@ -1309,6 +1309,11 @@ describe('asset references', () => {
       expect(storage.verifyAssetFile(signedFile)).toBe(true)
     }
     expect(content.footer.file).toBeNull()
+    // Sign copies, leaving the files of the record untouched:
+    for (const loadedFile of [header.file, ...gallery.files]) {
+      expect(loadedFile).toBeInstanceOf(AssetFile)
+      expect(loadedFile).not.toHaveProperty('signature')
+    }
   })
 
   it('recounts each location of overlapping data paths once', async () => {