I posted my slides on my web as a pptx file a few weeks ago. Suddenly after that, students gave some comment that when they downloaded the file, the file came as .zip. Even though the file on the server is not zipped and is name exactly as .pptx. Of course, I have tested my upload, but only on Firefox, which definitely has no problem. From the student comment, I tried it on IE and managed to duplicate the error. My first thought is that it must be something about the MIME type. The problem is I don't know the correct mime type of the pptx file.
Googling around leads me to this post on Drupal website. In short, I have to add these lines to /etc/mime.types so that Apache sends the appropriate MIME type to IE7. I wonder why IE bothers to change the file extension after all.
application/vnd.ms-word.document.macroEnabled.12 docm
application/vnd.openxmlformats-officedocument.wordprocessingml.document docx
application/vnd.openxmlformats-officedocument.wordprocessingml.template dotx
application/vnd.ms-powerpoint.template.macroEnabled.12 potm
application/vnd.openxmlformats-officedocument.presentationml.template potx
application/vnd.ms-powerpoint.addin.macroEnabled.12 ppam
application/vnd.ms-powerpoint.slideshow.macroEnabled.12 ppsm
application/vnd.openxmlformats-officedocument.presentationml.slideshow ppsx
application/vnd.ms-powerpoint.presentation.macroEnabled.12 pptm
application/vnd.openxmlformats-officedocument.presentationml.presentation pptx
application/vnd.ms-excel.addin.macroEnabled.12 xlam
application/vnd.ms-excel.sheet.binary.macroEnabled.12 xlsb
application/vnd.ms-excel.sheet.macroEnabled.12 xlsm
application/vnd.openxmlformats-officedocument.spreadsheetml.sheet xlsx
application/vnd.ms-excel.template.macroEnabled.12 xltm
application/vnd.openxmlformats-officedocument.spreadsheetml.template xltx