the problem is that right now the app looks for the file extension in the URL of the files, so if a URL doesn't contain the file extension then it's going to be downloaded without extension. It was implemented like this to make it faster, simpler and decrease the network usage. We have an open issue to improve it:
New Moodle versions should automatically add the extension of a file when you upload it, or at least it does it for me.
In your case, the URL to the file doesn't contain the extension. Android is able to open the file without having the extension (at least in my phone), but iOS can't.
As a temporary solution, you could modify the file to add the extension to the file name, that way it should work in the Moodle Mobile app.