Issue Description
The renewed certificate downloads from identity.apple.com as a .cer file, which is not a file format that Jamf Pro accepts for this process.
This can be browser related. It seems that the file extension may be changed by the Chrome browser itself when it's 'Ask where to save each file before downloading' feature is enabled.
Troubleshooting Steps
Option 1: Use Safari or a different browser to download the renewed Push certificate from the Push Portal.
Option 2: If experiencing the problem with Chrome (or similar browser type), go to the Settings section of Chrome and search for 'Ask where to save each file before downloading' in the 'search settings' bar. Disable the setting and try the renewal again.