Do high heels increase female attractiveness in men only through indirect means?
It is probably quite simple to test whether high heels themselves are the main cause of the increase in attractiveness, or rather the changes to posture imposed by them - just let participants rate the attractiveness of women who do not wear high-heels, but, for instance, hold them in their hand. 

Alternatively, you could expose one half of your participants to high heels and the other half not. Afterwards let them rate pictures of women and compare the mean ratings of the groups. 

In principle, you could a multitude of experiments with women and high heels - as long as the women do not wear the high heels, you can exclude that the effect on posture explains your results. 

An alternative approach, suggested by Artem Kaznatcheev in the comments, would be to compare the attractiveness ratings on women with and without high heels while their feet are not visible (e.g., covered or removed from the picture). Finding the same rating difference like observed with visible feet would suggest an effect of posture caused by high heels, rather than high heels themselves. (Actually you would need at least an two-by-two design ([high-heel vs. no-high-heel] x [feet-visble vs. feet-invisble]) to draw this conclusion.)