D = dlmread('voltage_u.txt','\t', 4, 0);
D = D(1:end-1,1:2);
t = D(:,1);
V = D(:,2);
L = size(D,1);
tv = linspace(min(t), max(t), L);
Ts = tv(2)-tv(1);
Fs = 1/Ts;
Vv = resample(V, t, Fs);
Fn = Fs/2;
FVv = fft(Vv)/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
Your time-domain data are not regularly-sampled, so it is necessary to use the resample function to provide a uniformly-sampled vector in order for the fft function to produce the correct results.
Explore this at your leisure to understand how it works.