Does "Magic Numbers" weaken encryption?

I was wandering at superuser and I found this question: Compress and then encrypt, or vice-versa?

Nearly all files have a Magic Number at a certain position in them. So, I wonder if I can attack (to break encryption or reduce possiblities...) by guessing several popular file formats?

I am aware that my question is very general without mentioning any encryption method or file format but I'm also looking for a general answer.

Thanks!