The code to get your source file isn't working. The return for that block is an error, so trying to use it downstream will not work.
Assuming you can get the JS to download the file working, I see that your Postman shows the file key referencing a value of type File. I'm not an expert on Postman, but I would hazard to guess that when you upload a file, it is being transformed in the background into some sort of object (much as Retool does), where the file name, file data and other attributes of the file are turned into key value pairs within the object. In your Retool OpenAI REST query body, you have file as a string, which may need to be adjusted to handle the object that you (probably) need to pass.