The causes for this problem might be various.
1. The attachment might not have been uploaded correctly in the first place. You want to make sure to ask the person who sent the attachments to send them again.
2. Or it might be that the attachments download incorrectly to your PC, which corrupts the file and hence, the attachments cannot be open.
Both problems can be solved if you used an alternative web browser, i.e. Chrome, Firefox, etc and if you made sure your internet connection stays uninterrupted during the upload or, respectively, download process.