Make it easier to share FreeMarker files in threads (permitted attachment extensions)

It is currently very difficult to share FreeMarker files within a post/thread (when the data is too large to comfortably be added in a code snippet) due to the ftl, txt, and zip file extensions not being permitted.

For example, I was trying to share two FreeMarker files in a Developer Discussion thread, but got an error when trying to attach the two .ftl files.  I then renamed the files to have .txt extensions and tried attaching them, but got another error. Next, I added both of the .ftl files into a zip archive and tried to attach said zip file, but this resulted in an error as well.

I eventually had to host the two files within a GitHub repo and share hyperlinks to them.

It shouldn't be this hard to contribute FreeMarker content on Atlas.  Please add at least the ftl extension as a permitted attachment extension in Community Admin.