Understanding time-response of photodiode circuit (LTspice)
Your 'filtering' capacitor and scope impedance form a high pass filter which 'differentiates' the square wave to produce a spike on each edge that decays exponentially. However with a time constant of 10ms the effect on a 1us pulse is unnoticeable. 

What you are seeing is a low pass filter with a time constant of ~1.7us, caused by the internal Base-Collector capacitance of the phototransistor interacting with the load impedance. The rising edge is fast because the photo current is high enough to discharge this parasitic capacitor quickly, but when the current stops the capacitor can only recharge slowly through R1. 

A time constant of 1.7us implies a capacitance of 1.7us/100k = 17pF. LTspice's model of the 4N25 has this:-

.model NP NPN(Bf=610 Vaf=140 Ikf=15m Rc=1 Cjc=19p Cje=7p Cjs=7p C2=1e-15)


Cjc is the Base-Collector zero-bias depletion capacitance, which at 19pF is close to the value I calculated by 'eyeballing' the graph.