`UploadFileItem.filetype` should be defined as optional

When using the `RNFS.uploadFiles` method, there is a type definition named [`UploadFileItem` with field `filetype` where the docs say...](https://github.com/itinance/react-native-fs#uploadfilesoptions-uploadfileoptions--jobid-number-promise-promiseuploadresult-)
> if not defined it will get mimetype from filepath extension

However, the type defines that field as required so it must be defined. Can we please switch it to be optional? Otherwise, we have to explicitly specify the filetype.