Tagging a .zip file with Document Library Columns

I have a Document Library for which a documentType column is defined. When I upload any file except for .zip files, I have the opportunity to enter values into the documentType column, thus classifying my document. This is not the case with .zip files, only the Name(filename) column is displayed. Same behavior if I choose the Edit Properties option from the document/file menu. Is there any reason why library metadata columns are disabled for .zip files? How can I tag these columns for .zip files? Thanks.