In this case, use FilenameUtils.getExtension from Apache Commons IO

Here is an example of how to use it (you may specify either full path or just file name):

import org.apache.commons.io.FilenameUtils;

// ...

String ext1 = FilenameUtils.getExtension("/path/to/file/foo.txt"); // returns "txt"
String ext2 = FilenameUtils.getExtension("bar.exe"); // returns "exe"

Maven dependency:

<dependency>
  <groupId>commons-io</groupId>
  <artifactId>commons-io</artifactId>
  <version>2.6</version>
</dependency>

Gradle Groovy DSL

implementation 'commons-io:commons-io:2.6'

Gradle Kotlin DSL

implementation("commons-io:commons-io:2.6")

Others https://search.maven.org/artifact/commons-io/commons-io/2.6/jar

You cannot do that. The getContentType() method simpy:

Returns the value of the content-type header field.

which in most cases is (though there is no guarantee) related to the file extension/file type, for example application/pdf would mean there is a PDF file under that URL.

Each of the file types with extension you have listed (rar, mp4, txt, jpeg,mkv, zip, png, apk, mp3) have another structure. To do reliably what you want to do, you would have to first download the whole file and then check its type based on the contents.

A good example of a library you could use is Apache Tika.