tika.java

/* import Tika into a file with this */
import org.apache.tika.Tika;

/* 
  assume the file's content is in a string called 'file'
  you can detect a file's mime type with the following
  https://tika.apache.org/1.8/api/org/apache/tika/Tika.html#detect(byte[])
*/
String detectedFileType = (new Tika()).detect(file.getBytes())

if ("text/plain".equals(detectedFileType)) {
  // a match! the file is plain text
} else {
  // no match!
}