Normalization of the action in Special Relativity

The action for a massive point particle in Special Relativity is given as$$A =-mc^2\int d\tau,$$Where $\tau$ represents the proper time, and $m$ represents the (rest) mass. From what I could understand, the Action must not change with respect to the reference frame, and hence it can be written as $$something\int d\tau$$ but why must the something need to be proportional to mass?