On OS X, how do you change a file's "Kind"?

The file kind is usually set by the application that handles that file type in Launch Services. For most of them, this is the application opening a file type by default, in your case, Coda.

Applications specify file types in their Contents/Info.plist file. You can see and edit it by right-clicking the application bundle and selecting Show Package Contents.

It looks like the Panic folks messed up something. All file types they define are PlainTextType:

You can change the name of the file type by editing all Document Type Name entries in Property List Editor, or all CFBundleTypeName entries when editing the file in a normal text editor. Just replace what you want to appear as description instead.

Edit, save, and make sure Launch Services notices (restart, log out, or just move the application to a different folder and back again), and the result looks like this:

You need to repeat this whenever there's an update to the application, since those replace the Info.plist file you edited. Best to report a bug to the developers at Panic.

I had this happen when I upgraded to Office 2016 and removed Office 2011 — the Finder decided that .dot files (MS Word 97-2004 templates) were GraphViz DOT files, even though I didn't have GraphViz installed, and decided that Nisus Writer should be the default application.

I managed to fix the default application the easy way in the Finder (Get Info / Open With: / Change All…), but the file kind was still wrong. Per Daniel Beck's answer I checked the Info.plists of Word 2016 and Nisus Writer, but they both were correct.

I ended up having to rebuild the Launch Services registry with

/System/Library/Frameworks/CoreServices.framework/Versions/Current/Frameworks/LaunchServices.framework/Versions/Current/Support/lsregister -kill -r -domain local -domain system -domain user

(from the Terminal) and that finally fixed it.