feat: add uploadFromBuffer method to Upload plugin for backend file uploads

Author: ivictbor

adminforth/documentation/docs/tutorial/08-Plugins/05-upload.md

@@ -305,6 +305,49 @@ new UploadPlugin({
 > adminServeBaseUrl defines the public path prefix. If your AdminForth base URL is /admin, files will be accessible under /admin/static/source/<key>.
 
 
+## API
+
+### uploadFromBuffer
+
+In some cases you may want to upload a file directly from your backend (for example, a file generated by a background job or received from a webhook) without going through the browser. For this, the Upload plugin exposes the `uploadFromBuffer` method.
+
+You can code it as custom logic or you can simply reuse Upload plugin for this purpose as well.
+
+This method uploads a file from a Node.js `Buffer`, automatically creates a record in the corresponding resource, and returns both the stored file path and a preview URL.
+
+```ts title="./some-backend-service.ts"
+import { admin } from './admin'; // your AdminForth instance
+
+...
+plugins: [
+  new UploadPlugin({
+    id: 'my_reports_plugin', // unique identifier for your plugin instance
+    ....
+  })
+]
+...
+
+const plugin = admin.getPluginById('my_reports_plugin');
+
+const { path, previewUrl } = await plugin.uploadFromBuffer({
+  filename: 'report.pdf',
+  contentType: 'application/pdf',
+  buffer, // Node.js Buffer with file content
+  adminUser, // current admin user or system user
+  recordAttributes: {
+    title: 'Generated report',
+    listed: false,
+  },
+});
+```
+
+- `uploadFromBuffer` uses the configured storage adapter (S3, local, etc.) to store the file.
+- It automatically creates a new record in the resource and stores the file path into the column defined by `pathColumnName`, together with any extra `recordAttributes` you pass.
+- It returns an object `{ path, previewUrl }`, where `previewUrl` is the same URL that would be used for previews inside AdminForth.
+
+> ⚠️ It is not recommended to upload large files from the backend using `uploadFromBuffer`, because the entire file must go through your server memory and network. For large uploads you should prefer frontend presigned uploads directly to storage. You can find an example of presigned upload flow using upload plugin in the Rich editor plugin source code (Rich editor actually uses Upload plugin to upload images in edited content).
+
+
 
 ## Image generation