StrutsActionRegistryImpl.getAction(String path) maintains a very lenient rule for returning action paths, probably for supporting wildcard matching.
A proper wildcard matching logic should be implemented.
Steps to reproduce the issue
0) Deploy document-library-hook-1.0.0-SNAPSHOT.war (in document-library-hook.zip you can also find the source code)
1) Create a folder in Document & Media
2) Add a document (like a pdf) into the new folder
3) Use the Document & Media display in a page
4) Click on the folder created (here custom action is called, the one we want to override)
5) Click on the specific document (here default behavior should be called, instead it's going inside our hook action)