They should be pptx files. If you have a .zip file containing a _rels, docProps and ppt dir and a [Content_Types].xml in the root, you can rename the .zip to .pptx. If you don't have PowerPoint you can download the viewer from http://www.microsoft.com/en-us/download/details.aspx?id=13

Good luck,
Peter

 

edit: maybe your unzip app recursively unzips all zip files inside the zip file, a.pptx file is in fact a zipped file