Scanning and indexing files video
Naming
The method name load_Directory_Files is a bit odd. To follow general Java naming conventions, it would be something like loadDirectoryFiles. This is called camel case.
File filtering
You can use a FileFilter to accept or discard files for the
listFiles call, no need to do it yourself:
import java.io.File;
import java.io.FileFilter;
import java.util.List;
import java.util.Arrays;

public class VideoOrDirectoryFilter implements FileFilter {

    private static final List&lt;String&gt; ALLOWED_EXTENSIONS = Arrays.asList(
        &quot;.mp4&quot;,&quot;.ts&quot;,&quot;.mkv&quot;,&quot;.mov&quot;,&quot;.3gp&quot;,&quot;.mv2&quot;,&quot;.m4v&quot;,&quot;.webm&quot;,
        &quot;.mpeg1&quot;,&quot;.mpeg2&quot;,&quot;.mts&quot;,&quot;.ogm&quot;, &quot;.bup&quot;, &quot;.dv&quot;,&quot;.flv&quot;,
        &quot;.m1v&quot;,&quot;.m2ts&quot;,&quot;.mpeg4&quot;,&quot;.vlc&quot;,&quot;.3g2&quot;,&quot;.avi&quot;,&quot;.mpeg&quot;,
        &quot;.mpg&quot;,&quot;.wmv&quot;,&quot;.asf&quot;
    );

    @Override
    public boolean accept(File file) {
        return file.isDirectory() 
            || ALLOWED_EXTENSIONS.contains(file.getName().toLowerCase());
    }
}

Null check
In loadDirectoryFiles, you can return early if the listFiles call is null:
File[] files = directory.listFiles(new VideoOrDirectoryFilter());
if (files == null) return;

Looping
Instead of the for loop with an index variable, you could use a foreach loop. That makes for cleaner code, as you directly get the object in the list. Also, you don't need to check the length of the list before looping. If it's empty, the loop will be a no-op.
for (File file : files) {
    // Do your thing with file here
}

Reimplementation
Here's a reimplementation of the loadDirectoryFiles method with the above enhancements included.
public static void loadDirectoryFiles(File directory) {
    File[] files = directory.listFiles(new VideoOrDirectoryFilter());

    if (files == null) return;

    for (File file : files) {
        if (file.isDirectory()){
            loadDirectoryFiles(file);
        } else {            
            Constant.allMediaList.add(file);
            String fileName = file.getName();
            String size = Function.humanReadableByteCountSI(file.length());
            String duration = Function.getDurationFF(file.getAbsolutePath());
            String folderName = file.getParentFile().getName();
            String path = file.getAbsolutePath();
            Log.d(TAG, &quot;load_Directory_Files: &quot;+ duration + &quot;&quot; +size + &quot;&quot; + fileName);
            Constant.allMediaListWithModel.add(new AllVideoModel(fileName, path, folderName, size, duration));
        }
    }
}