WordPress by default will preserve capitalisation in filename for images. To make filenames lowercase automatically, add the following to the functions.php of your active theme.

/**
 * Convert filename to lowercase
 *
 * @author Arun Basil Lal
 * @refer https://codex.wordpress.org/Plugin_API/Filter_Reference/wp_handle_upload_prefilter
 */
function prefix_convert_filename_to_lowercase( $file ) {
	
	$image_extensions = array (
		'image/jpeg', 'image/gif', 'image/png', 'image/bmp', 'image/tiff', 'ico'
	);
	
	// Return if file is not an image file
	if ( ! in_array($file['type'],$image_extensions) ) {
		return $file;
	}
	
	$image_extension = pathinfo( $file['name'] );
	$image_name 	 = strtolower( $image_extension['filename'] );
	$file['name'] 	 = $image_name . '.' . $image_extension['extension'];
	
	return $file;
}
add_filter('wp_handle_upload_prefilter', 'prefix_convert_filename_to_lowercase', 20 );