SELECT
generatedFile.FileName AS "GeneratedFileName",
( SELECT Files.FileName
  FROM Files
  WHERE Files.Id = DocumentTemplates.FileId
) AS "InitialFileName",
REPLACE(generatedFile.FileName,
( SELECT Files.FileName
  FROM Files
  WHERE Files.Id = DocumentTemplates.FileId
),'') AS "NeededName"
FROM WorkDocuments
JOIN DocumentTemplates ON WorkDocuments.SourceDocumentTemplateId = DocumentTemplates.Id
JOIN Files generatedFile ON generatedFile.Id = WorkDocuments.FileId