As a charge is accelerated by a force and increases its speed, it
increases its mass relativistically.Since the ratio of E / B determines
its speed, B could decrease as the mass increases.This B must be a very short range magnetic field which in a photon is equal in magnitude to the
usual magnetic field strength of the photon at a particular point.
To be short range the magnetic field must have very high energy and therefore it must be associated with a lot of mass.When a charge increases its mass relativistically, perhaps it gets the relativistic mass from the
short range magnetic field which gets weaker in strength with increasing speed.