Is it possible to rename a field (document file name)?

I have a form that i am uploading some files vie the dropBox API.

The problem is that if i upload a document in Greek characters like: ελλάδα123.png the file is being uploaded like this 123.png. Ι have already talked to the support and they are still don’t know why this happens…

But i found a shortcut, if i upload the document like this f_ελλάδα123.png everything works fine.
It appears that if you place an english letter first everything works fine. The problem here is, how can i rename the field (Document file name) ???

This is not working:
{fileName} = "f_" . {fileName}; // This returns an error file that doesn’t exist, so do you have any idea of how to fix this?