Hey @Panguino and @rodrigowpl 
I’m sorry to hear you were facing some issues uploading a file to monday.com through Postman. Let’s figure this out together!
It seems like your setup is almost right! That said, our API will not accept “operations”, and will instead expect query to hold your actual API call. It also seems you are using an endpoint https://api.monday.com/v2/files, would you be able to change that to the following endpoint instead?
https://api.monday.com/v2/file
You can also find out a bit more about using variables in this API call in this community post:
Announcement: Uploading files to monday.com has just become easier
Additionally, feel free to take a look at a basic example of structuring the API call in the same using NodeJS below:
Uploading a file to monday.com with variables - example in NodeJS
I hope this is helpful 
Let me know how things worked out!
-Alex