Assume that the value of an integral is real

First, let's look at what happens if we factor the ep term out of the integral, like this

ClearAll[a, b, f, R, t]

ep = a + I b;

B = Integrate[f[t] , {t, -R, R}] ep;

With[{$Assumptions = Element[Integrate[ f[t], {t, -R, R}], Reals]},
 Re[B] // ComplexExpand // Simplify
 ]

(*   a*Integrate[f[t], {t, -R, R}]   *) 

So, it looks like MMA is able to apply the assumption that the integral is real. Note that we used With to make a temporary change to the global $Assumptions and we applied the assumptions using Simplify. (The Simplify takes about 12 seconds on my desktop. I wonder why.)

Next, we start with the ep term inside the integral. This time we will use With to set the $Assumptions and to factor ep, or any other contants, out of the integral, like this

ClearAll[a, b, f, R, t]

ep = a + I b;

B = Integrate[f[t] ep, {t, -R, R}]

With[{$Assumptions = Im@Integrate[ f[t], {t, -R, R}] == 0,
  B = B //.
    Integrate[q1___ r__ q2___, {v_, s___}] /; FreeQ[{r}, v] :>
     r Integrate[q1 q2, {v, s}]},
 
 (Re[B] // ComplexExpand // Simplify) /.
  Times[r_, Integrate[q_, {v_, s___}]] /; FreeQ[{r}, v] :> 
   Integrate[r q, {v, s}]
 ]

(*   Integrate[a*f[t], {t, -R, R}]   *) 

Note the different, but equivalent, assumptions in the two With statements. Also note that factoring the constants out of the integral is done with code provided by @dr-belisarius in his answer to How to do algebra on unevaluated integrals?. Another reference that may be useful is How to simplify symbolic integration.