In a previous PR, @fabpot says that he prefered to avoid adding another parameter (https://github.com/symfony/symfony/pull/10917#issuecomment-43341918).
And I agree with him, the purpose of this new parameter will just be to let the user choise the format of the error message :confused: 

But you're right this trick is trying to avoid to expose a new paramater, but in the reality, there is a new parameter. 

Should I leave maxSize has the user defined it. And expose 2 getter in Constraint\File: getMaxSizeBytes and getMaxSizeFormat ? But this is not what @webmozart  expected in his ticket #10962