Extract can methods

This is a minor refactoring that possibly helps to improve the readabiity of the mapping files to resources.

Actually, I am still a bit confused about what mappings are possible:

* single file --> single resource (any file or specific file)
* directory --> index resource (default path)

If this is correct, maybe it would help to extract one more method: `_canSingleFile` or similar. Then, the main mapping function would become:

* _canSingleFile
* _canDirectory

And it would be clearer to me how it works. What do you think?