Hi Ali Al-Ammar,
The possibilities regarding uploading files in ATS depend on what kind of selenium setup you are using. If you have a setup with browserstack you can use default testfiles that browserstack offers. In ATS you can record uploading a file and it records all the actions. After recording edit the upload acton and change the filepath in one of the default filepaths that are offered by browserstack. I suppose it works kinda the same way when you use a different saas selenium setup.
More information about the different setups and file uploading in ats can be found here: