docs: update file-upload.md

Add a notion about the possibility to make the file presence optional

Author: CodingWatchCollector

content/techniques/file-upload.md

@@ -155,6 +155,8 @@ Finally, you can use the special `ParseFilePipeBuilder` class that lets you comp
 file: Express.Multer.File,
 ```
 
+> info **Hint** File presence is required by default, but you can make it optional by adding `fileIsRequired: false` parameter inside `build` function options (at the same level as `errorHttpStatusCode`).
+
 #### Array of files
 
 To upload an array of files (identified with a single field name), use the `FilesInterceptor()` decorator (note the plural **Files** in the decorator name). This decorator takes three arguments: