weird problems with file uploads

Hi all! This is my first intervention in this list. This is probably a tyipical problem but I have scoured the web for a solution and I haven't found one. I'm trying to implement a file upload on a website so people can send us files of their work and I'm having a fantastic success with text files. The problem comes when I try to upload binary files such as zip or mp3, when I get the typical file attack error mesage from the script, in other words, the is_uploaded_file() function does not recognise the file correctly. can anyone give me a lead as on what the problem might be? Thanks for your patience Jordi