Datus is probably the strongest "file extension detector and metadata extractor". It is based on both "Apache Tika project" and "drewnoakes's metadata-extractor". The Apache Tika™ toolkit detects and extracts metadata and text from over a thousand different file types (such as PPT, XLS, and PDF) based on their content rather than just name. Tika solution and metadata-extractor were successfully used as back-end in this very simple Android application. It does not connect to any server; So it is 100% offline.