Mendix has a guide about ATS here: https://docs.mendix.com/ats/
This should probably help you with the first steps.
Good luck :)
Uploading files with ATS is possible, but complicated. Not because we don't want you to upload files, but because of the following:
When you run a test case in ATS, a Browser will be opened at your Selenium server/service, so not on your local machine. If you use the file manager widget and set a file location there, it will be the location relative to the environment the browser is running on. That means that the file you want to upload should be on the same server as Selenium. You need to put your file(s) there yourself. For a local (custom) Selenium server, this can of course easily be done, but Browserstack and Saucelabs do not currently support this. Browserstack does have some kin of workaround, if you could download the file you want to import from the/a Mendix app as well, that file will be stored (temporarily) at Browserstack, and you can set the file manager to the file in the download directory.
The file paths are dependent on the OS on the remote machine. The download paths are as below.
C:\Documents and Settings\hello\Downloads
Win 7 / 8 / 8.1 / 10:
The only other option that Browserstack has for uploading files, is to make use of their test files, as described in:
Hopefully this will help you on your way. If not, please contact me and I'll try to help you.
With kind regards,
Joris van der Heijden
ATS Consultant @ Mansystems