I can't do anything against that you don't see the problem. Just don't be surprised that a function that needs a string to function does not work if there is no string.
Your description
is absolutely correct: you have to store the value first and then you can execute more or less every action/manipulation on it.
But you said already
The already working method is to use the admittedly in this case more complex action "Guess value"
"Guess value" takes more or less any string and not just the filename for convenience reasons and then allows to prepare that string so that the bits and pieces can be assigned to (other) fields.
So, the only method for the time being, until your feature request is implemented is either go the way with several steps with the existing Convert>Filename-Tag and then further actions or fewer steps with an action of the type "Guess value".which requires a more complex preparation.