diff --git a/src/servers/api/file-browser/router.ts b/src/servers/api/file-browser/router.ts
index 70c2d4ea..793d9d73 100644
--- a/src/servers/api/file-browser/router.ts
+++ b/src/servers/api/file-browser/router.ts
@@ -433,6 +433,16 @@ router.post('/rename', async (ctx) => {
  *
  * The cache key contains the file's mtime and size, so this URL's answer genuinely cannot change: a
  * different file is a different URL. That is what makes a year-long cache correct rather than a bet.
+ *
+ * It was only ever half true, and the half that was missing is the half a browser can see. The DISK key
+ * has always held mtime and size, so the server regenerated correctly every time. The URL held the path
+ * and the box size and nothing else — so a browser told to keep the answer for a year kept it for a
+ * year, and a file replaced or renamed into a name already drawn showed the picture that used to be
+ * there. Nothing cleared it but somebody who knew to hard-reload.
+ *
+ * The client now sends `v=<mtime>-<size>` — see `getThumbUrl`. Nothing here reads it: this route stats
+ * the file anyway, and a parameter the server depended on would be a second source of truth about which
+ * file is meant. It is in the URL to be part of the URL, which is the whole job.
  */
 router.get('/thumb', async (ctx) => {
   const user = ctx.get('user');
diff --git a/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/FileItem.tsx b/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/FileItem.tsx
index ab93be59..7b42b9bc 100644
--- a/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/FileItem.tsx
+++ b/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/FileItem.tsx
@@ -52,7 +52,7 @@ import { cardStyle } from '@/components/Card';
 import type { DirEntry } from '../../../../hooks/useFilesAPI';
 import type { TaskGroup, TaskSummary } from '../../useTasks';
 import type { AgentGroup, AgentSummary } from '../../useAgents';
-import { getFileType } from '../../../FileViewer';
+import { getFileType, thumbVersion } from '../../../FileViewer';
 import { useTranslation } from '../../../../locales';
 import { useFormat } from 'i18n';
 
@@ -670,7 +670,16 @@ export const FileItem = ({
     rounded box would trim content off a photograph for decoration.
   */
   const withThumb = (kind: 'image' | 'video', node: React.ReactNode, className: string, size: number) => (
-    <Thumbnail path={path} root={root} size={size} isVideo={kind === 'video'} className={className}>
+    <Thumbnail
+      path={path}
+      root={root}
+      size={size}
+      // Which version of the file, so replacing or renaming one does not leave the old picture in a
+      // browser cache that was told it could keep it for a year. See `Thumbnail`.
+      version={thumbVersion(entry)}
+      isVideo={kind === 'video'}
+      className={className}
+    >
       {node}
     </Thumbnail>
   );
diff --git a/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/Thumbnail.tsx b/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/Thumbnail.tsx
index e92c1bf7..64e2366b 100644
--- a/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/Thumbnail.tsx
+++ b/src/workspaces/officerdev/src/apps/FileBrowser/FileBrowserApp/components/Thumbnail.tsx
@@ -52,6 +52,7 @@ export const Thumbnail = ({
   path,
   root,
   size,
+  version,
   isVideo,
   className,
   children,
@@ -59,6 +60,16 @@ export const Thumbnail = ({
   /** Root-relative, the same path every other file-browser call uses. */
   path: string;
   root?: string;
+  /**
+   * Which VERSION of the file this is — `mtime-size`, from the listing.
+   *
+   * The response is `immutable` for a year, and the URL used to carry only the path, so a browser that
+   * had drawn a thumbnail for a name went on drawing it however the file behind that name changed. A
+   * rename into a name already seen showed the picture that used to be there, and nothing short of a
+   * hard reload cleared it. The SERVER was never wrong — its own cache key has always held mtime and
+   * size — the URL simply did not say which file it meant. See `getThumbUrl`.
+   */
+  version?: string;
   /** What to ask the server for. Snapped server-side, so a near-enough number is fine. */
   size: number;
   /** Draws the play badge. The thumbnail of a video is a still, and a still is not obviously a video. */
@@ -79,7 +90,8 @@ export const Thumbnail = ({
 
   const token = getHeaders()['Authorization']?.replace('Bearer ', '') ?? '';
   const rootParam = root && root !== 'home' ? `&root=${encodeURIComponent(root)}` : '';
-  const src = `${API_URL}/file-browser/thumb?path=${encodeURIComponent(path)}&size=${size}&token=${encodeURIComponent(token)}${rootParam}`;
+  const versionParam = version ? `&v=${encodeURIComponent(version)}` : '';
+  const src = `${API_URL}/file-browser/thumb?path=${encodeURIComponent(path)}&size=${size}&token=${encodeURIComponent(token)}${rootParam}${versionParam}`;
 
   return (
     <span className={`relative inline-flex items-center justify-center overflow-hidden ${className ?? ''}`}>
diff --git a/src/workspaces/officerdev/src/apps/FileViewer/FileCarousel.tsx b/src/workspaces/officerdev/src/apps/FileViewer/FileCarousel.tsx
index cb0744df..b6aed840 100644
--- a/src/workspaces/officerdev/src/apps/FileViewer/FileCarousel.tsx
+++ b/src/workspaces/officerdev/src/apps/FileViewer/FileCarousel.tsx
@@ -1,7 +1,7 @@
 import { useCallback, useEffect, useMemo, useState } from 'react';
 import { ChevronLeft, ChevronRight } from 'lucide-react';
 import { useFilesAPI, type DirEntry } from '../../hooks/useFilesAPI';
-import { getFileType, getThumbUrl } from './file-types';
+import { getFileType, getThumbUrl, thumbVersion } from './file-types';
 import { useFileViewer } from './FileViewerContext';
 
 /**
@@ -128,7 +128,7 @@ export const FileCarousel = ({ onClose }: { onClose?: () => void }) => {
             >
               {entry.thumb ? (
                 <img
-                  src={getThumbUrl(path, root)}
+                  src={getThumbUrl(path, root, 256, thumbVersion(entry))}
                   alt=""
                   loading="lazy"
                   decoding="async"
diff --git a/src/workspaces/officerdev/src/apps/FileViewer/file-types.ts b/src/workspaces/officerdev/src/apps/FileViewer/file-types.ts
index 66baedd0..f1484211 100644
--- a/src/workspaces/officerdev/src/apps/FileViewer/file-types.ts
+++ b/src/workspaces/officerdev/src/apps/FileViewer/file-types.ts
@@ -165,14 +165,31 @@ export function getRawUrl(filePath: string, root?: string, audioTrack?: number):
  *
  * Same route, same cache, same size — so a folder already browsed has a warm carousel strip, and a
  * format that needs a plugin to decode works here exactly where it works there.
+ *
+ * `version` is what makes the year-long `immutable` on that response honest. The SERVER's cache key has
+ * always held the file's mtime and size; this URL held only the path, so a browser that had once drawn
+ * a thumbnail for a name kept drawing it for a year however the file behind that name changed. Upload,
+ * rename something else into the name, replace a photograph — the picture on screen stayed. Pass
+ * `mtime-size` and a changed file is simply a different URL. The server ignores the parameter; it stats
+ * the file regardless.
  */
-export function getThumbUrl(filePath: string, root?: string, size = 256): string {
+export function getThumbUrl(filePath: string, root?: string, size = 256, version?: string): string {
   const headers = getHeaders();
   const token = headers['Authorization']?.replace('Bearer ', '') ?? '';
   const rootParam = root && root !== 'home' ? `&root=${encodeURIComponent(root)}` : '';
-  return `${API_URL}/file-browser/thumb?path=${encodeURIComponent(filePath)}&size=${size}&token=${encodeURIComponent(token)}${rootParam}`;
+  const versionParam = version ? `&v=${encodeURIComponent(version)}` : '';
+  return `${API_URL}/file-browser/thumb?path=${encodeURIComponent(filePath)}&size=${size}&token=${encodeURIComponent(token)}${rootParam}${versionParam}`;
 }
 
+/**
+ * What identifies this VERSION of a file, for a cache key — the same two facts the server hashes.
+ *
+ * One place, because the grid and the carousel strip must agree: two spellings would mean two URLs for
+ * one file, which is two renders and two cache entries for no reason.
+ */
+export const thumbVersion = (entry: { modifiedAt?: number; size?: number }): string | undefined =>
+  Number.isFinite(entry.modifiedAt) && Number.isFinite(entry.size) ? `${entry.modifiedAt}-${entry.size}` : undefined;
+
 export function getAudioTracksUrl(filePath: string, root?: string): string {
   const headers = getHeaders();
   const token = headers['Authorization']?.replace('Bearer ', '') ?? '';
diff --git a/src/workspaces/officerdev/src/apps/FileViewer/index.ts b/src/workspaces/officerdev/src/apps/FileViewer/index.ts
index 3a0c7dd7..290a58b6 100644
--- a/src/workspaces/officerdev/src/apps/FileViewer/index.ts
+++ b/src/workspaces/officerdev/src/apps/FileViewer/index.ts
@@ -4,7 +4,7 @@ export { FileViewerProvider } from './FileViewerProvider';
 export { FileViewerHeader } from './FileViewerHeader';
 export { FileViewerBody } from './FileViewerBody';
 export { useFileViewer } from './FileViewerContext';
-export { getFileType, getLang, getExt, getArchiveBaseName, ARCHIVE_EXTS, type FileType } from './file-types';
+export { getFileType, getLang, getExt, getArchiveBaseName, thumbVersion, ARCHIVE_EXTS, type FileType } from './file-types';
 
 // No `appRegistryMetas` here on purpose. The file viewer is not a panel app you can put on a dashboard:
 // it is mounted as an ephemeral panel by `useFileViewerPanels`, which supplies `FileViewerBody`/`Header`