Only documents (word docs, txt, etc), images, videos, and audio files display. It is also not possible to traverse many types of directory structures such as a folder inside the downloads folder.
The only way (as far as I can tell) to import a dictionary on android is to rename its extension to one of the file types it finds and make sure it is in a top level default directory. For example, renaming some_dictionary.zip to some_dictionary.png and placing it into the Download folder.