public class CustomFileSystemProviderBuilder :
    FileSystemProviderBuilder

@(Html.DevExtreme().FileManager()
    .FileSystemProvider(provider => provider.Custom()
        .GetItems("getItems")
        .UploadFileChunk("uploadFileChunk")
        .DownloadItems("downloadItems")
    )
    //...
)
<script>
    function getItems(parentDirectory) {
        //your code
    }

    function uploadFileChunk(fileData, uploadInfo, destinationDirectory) {
        // your code
    }

    function downloadItems(items) {
        // your code
    }
</script>