Mystique replicating target's attire, how does it work?

I have always wondered how does Mystique's transformation works.

Please spare me as I don't read many Marvel comics, just those random ones which I can get.

From what is seen in movie, she can instantly transform into anyone once she gets clear look at him/her. She can even replicate the target's attire. In this case, she doesn't know what he/she looks under the attire, so does she just copies the dimensions to match the size while remaining blue skinned under it?