Complex Projection combining part of two fields

Summary:

I am currently extracting attached files information from transitions. I would like to create as a part of my extract a new field by combining two fields.

Content (required):

AttachedFiles,FileContent Will give me a full file name. e.g. PassportPhoto.pdf

AttachedFiles,FileCategory,Code Will give me the uploaded file category e.g. IDfile

I want a new complex projection that will give me a combination of the file category and the file extension. e.g. IDFile.pdf

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):