How can I store the exact size of the file being uploaded in a variable?

I am facing a problem where I want to store the exact size of an uploading file in a variable so that I can access it later to validate even if the section containing the file upload field is hidden. I am on v19.1. PS. By size, I mean the exact size in bytes and not "<1KB" which is retrieved by using fv!files.size. I cannot use a!save() function in validation attribute and fv!files.size is not accessible outside validation.

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