Of course, filler dramatically influence on mechanical properties of materials including tensile and flexural strength. As a rule, filler has more high modulus of elasticity and strength than matrix material.  Shortly saying, fillers plays reinforcing function  because they carry out stresses in the loaded material by way "elastic matrix - rigid filler".  So, until some limiting content (depending on adhesion between matrix and filling) increasing of filler concentration gives us growth  in strength in comparison to base (unfilled) material.  

It will depend on the properties, size and quantity of filler materials added. 

Fillers increase the elastic properties of composites , provided a good adhesion with the matrix is in play.  Halpin-Tsai equation is a good model to predict the  elastic properties of of particulate composites. Anyway at moderate filler content it predicts the same results of a "series" model:

Ec=(Em*Ef)/(Vf*Em+Vm*Ef)

Where Ec, Em, Ef are the moduli of composite, matrix and filler, respectively and Vf and Vm the volume fractions with Vf+Vm=1

The strength of composites is matrix-dominated. In general the strength of filled composites decreases with the filler content.

However the strength is statistical function , not a number.

When the strength is treated with a Weibull cumulative distribution function (https://en.wikipedia.org/wiki/Weibull_distribution)

one generally observes both a decrese of the characteristic strength (the scale factor)  and a decrease of the shape factor of the strength distribution function, with increasing the filler content.

Fillers of-course increase strength in composites,but mainly they act as shock absorbers, dampen vibrations & provide structural stability.Has to be analyzed by equations

of course, the addition of filler improves the properties of polymer composites to a certain limit and it depends on the quantity and size of fillers used