docs(openapi): Add example for multiple files uploading

Author: duongleh

content/openapi/operations.md

@@ -136,6 +136,14 @@ class FileUploadDto {
 }
 ```
 
+To handle multiple files uploading, you can define `FileUploadDto` as follows:
+```
+class FileUploadDto {
+  @ApiProperty({ type: 'array', items: { type: 'string', format: 'binary' } })
+  file: any;
+}
+```
+
 #### Extensions
 
 To add an Extension to a request use the `@ApiExtension()` decorator. The extension name must be prefixed with `x-`.