Uploads conspiracy or curiosity?

I am puzzled. I have built a php script to upload a document to a directory and save the info in an MySQL table. I created a sample document in OpenOffice and saved it as a .doc file. When I access the page via an Opera browser, I can upload the file without problem. When I access the page via MS Explorer, the document simply does not upload. There are no error messages. However, if I attempt to upload another file type, such as a PDF, Explorer uploads it without problem. In doing this, I recall some curious behaviour with Explorer uploading OpenOffice documents saved as OpenOffice documents in the past. If I were conspiracy minded, I'd suspect that this is MS's way of discouraging use of OpenOffice. In any event, I am curious to know what is causing this. Thanks, Jeffrey Baumgartner