Actually, what I believe will solve this issue is a regex pattern that I can use with the "Apply a Regular Expression" action to retrieve the file extension of a file name.

I can retrieve the name of the file from Salesforce in the workflow. However, my attempts at a regex pattern to retrieve the extension names have been unsuccessful. If I can get the file extension and store it in a workflow variable, then I can simply add that to the end of the file name when I rename it in the workflow! 

I've looked at a few regex patterns from google but some of the results I've found mention that they seem to be javascript specifc, or python specific etc. I'm not sure what type of pattern would work in the Nintex Workflow action. Any help would be greatly appreciated!

Well just in case anyone comes across this in the future, I was able to use the 'Retrieve a record' action to retrieve the attachment record from Salesforce. Then, with the attachment file name, I was able to extract the extension with this regex pattern: .[0-9a-z]+$

Ex filename - Testfile.pdf -> the Regex pattern will output ---> .pdf

Nintex regex follows the .Net format for regular expressions. After that, simply store the file extension as a variable and add it to the the end of whatever text is being used to re-name the file!!!