Flush image styles when replace method is used

Closes #3420449

src/Plugin/migrate/process/FileImport.php

@@ -248,6 +248,14 @@ class FileImport extends FileCopy {
         // If this was a replace, there should be an existing file entity for it
         // And if so, we return it. Otherwise, one will be created further down.
         if ($file = $this->getExistingFileEntity($final_destination)) {
+          // If this was a replace, need to flush all its image styles,
+          // otherwise they don't regenerate with the updated image content.
+          if (
+            $this->configuration['file_exists'] === FileSystemInterface::EXISTS_REPLACE ||
+            (enum_exists(FileExists::class) && $this->configuration['file_exists'] === FileExists::Replace)
+          ) {
+            image_path_flush($file->getFileUri());
+          }
           return $id_only ? $file->id() : ['target_id' => $file->id()];
         }
       }