[support] all attachments's mimetype in plain/text?

Hi, it is very strange. I'm using a module called disknode for file managment, because it is the only one that allows admin or users to select a file to attach to a node that is already uploaded into the server. Since 2 persons say that using firefox, instead of downloading a zip file, they see all the characters and strange symbols as a text file, i clicked the info button of disknode of a zip file, and it says, mimetype=plain/text. I don't think the problem is given by disknode, because it says the same thing plain/text mimetype also if I click the info button on a file that I uploaded with the simple upload module of drupal. I don't know from where to start to solve this problem... Any ideas? thanks