minor #7981 Update upload_file.rst (rumours86)

This PR was submitted for the 3.3 branch but it was merged into the 2.7 branch instead (closes #7981).

Discussion
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Update upload_file.rst

If you edit an existing product with an attached file and do not fill in the fields for the file, after saving the field with the file is reset

Commits
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d015048 Update upload_file.rst

Author: xabbuh

controller/upload_file.rst

@@ -332,6 +332,7 @@ automatically upload the file when persisting the entity::
     class BrochureUploadListener
     {
         private $uploader;
+        private $fileName;
 
         public function __construct(FileUploader $uploader)
         {
@@ -362,11 +363,10 @@ automatically upload the file when persisting the entity::
             $file = $entity->getBrochure();
 
             // only upload new files
-            if (!$file instanceof UploadedFile) {
-                return;
+            if ($file instanceof UploadedFile) {
+                $fileName = $this->uploader->upload($file);
             }
 
-            $fileName = $this->uploader->upload($file);
             $entity->setBrochure($fileName);
         }
     }
@@ -449,8 +449,9 @@ controller.
                     return;
                 }
 
-                if ($fileName = $entity->getBrochure()) {
-                    $entity->setBrochure(new File($this->uploader->getTargetDir().'/'.$fileName));
+                $this->fileName = $entity->getBrochure()
+                if ($this->fileName) {
+                    $entity->setBrochure(new File($this->uploader->getTargetDir().'/'.$this->fileName));
                 }
             }
         }