We are unable to set the File Type Association for Client to Host Redirection to work on a specific application. The FTA list would remain empty even though the extension(s) was listed under HKEY_CLASSES_ROOT in the registry. Even setting the file extensions default program in the Control Panel > Default Programs > Set Associations still would not bring up the default FTAs in Studio.

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

Registry keys for the FTAs under HKEY_CLASSES_ROOT were missing. After manually adding them, Studio is able to see the FTAs for the applications

To fix this, we set the registry keys so Studio could associate the file types as expected: (This is to be done on the Server VDA Hosting the application) 

***Note: Make sure you create a snapshot or backup of the Server VDA before proceeding****

Open the Registry > Navigate to HKEY_CLASSES_ROOT > Search for the file extension in question. For this case it was .pvz:

Under .pvz key in the Default key as shown above, it displays the actual extension seen by Windows. Now search for this key in the HKEY_CLASSES_ROOT: PTCProductView.pvzfile 

Once you find the key, expand it and click on "Shell" and under Default key enter "open" as shown below:

Now right-click on Shell and create a new key named "open". Right click on "open" and create a new key named "command":

Inside the command key, in the Default value add the path to the application in question.

Once this is complete, navigate to Studio > Application Properties (App in question) > File Type Association, and now you see the available file type. This must be done with all the file types needed to see them all.

Or you could open command prompt and run the following:
ftype "Actual File Type seen by Windows" and it will output the location of the application that can open that file type. For this case we used the following: ftype ptcproductview.pvzfile

This article helps explain how to manually set file type extensions for applications when studio is unable to find it by default.