I've discovered that this error is related with this issue:
It's a file extension related problem. With new files we're able to detect the right extension, but the old ones return a different value than the one expected. In the issue above we have improved the download system, and now it works for both files. This will be solved in the next release (September).
For files already downloaded with this problem, the first time the user opens the file he won't see the share button yet because the file needs to be redownloaded. The file will be automatically redownloaded in the background when the user clicks to open the file. Once the file has been redownloaded the problem should be fixed if he opens the file again.