When you are in the package that “owns” the FileHeader, you can use dot-notation to access the field, e.g. fh.content, if you are not the owner of that struct you can not access the field and have to take a look at the documentation of the owning package if there are functions or methods to extract the fields value.
PS: If you are talking about multipart.FileHeader, I can’t see a possibility to access that field on a quick glance, I didn’t delve the code though, perhaps its possible by "Open()ing it?