I am trying to add new field to user profile, or rather image for avatar
I created new simple file

class MemberExtension extends DataExtension {
	static $db = array(
        'Avatar' => 'Varchar(200)',
    );

	function updateCMSFields(FieldList $fields) {
		$fields->addFieldToTab("Root.Main", new UploadField('Avatar'));
	}
}

Object::add_extension("Member", "MemberExtension"); 

into _config.php
After that my CMS just got crazy.
I cant enter to Files or any else section. I cant upload image (got some Json error).
What i do wrong?

UploadField expects a has_one relationship to the image class.

@see
http://doc.silverstripe.org/framework/en/reference/uploadfield

class MemberExtension extends DataExtension {
	static $db = array(
        'Avatar' => 'Varchar(200)',
    );
	static $has_one = array(
        'AvatarImage' => 'Avatar'
    );
	function updateCMSFields(FieldList $fields) {
		$fields->addFieldToTab("Root.Main", new UploadField($name = 'AvatarImage',$title = 'Avatar image'));
	}
}

$has_one not helps.
And by the way there are no php errors in the log.
Any ideas?

As I've said, you need a relationship to a image class.

class MemberExtension extends DataExtension {
	static $has_one = array(
		'Avatar' => 'Image',
	);
	function updateCMSFields(FieldList $fields) {
		$fields->addFieldToTab("Root.Main", new UploadField('Avatar'));
	}
}

@see
http://doc.silverstripe.org/framework/en/reference/uploadfield

there are no php errors in the log

You need to set your enviroment to 'dev mode' for proper error messages.
Director::set_environment_type("dev");

@see
http://doc.silverstripe.org/framework/en/topics/debugging

it was to no purpose
I have Director::set_environment_type("dev"); in my _config.php

After login to CMS it jumps to Page section. If I press "Files" or eny else section, loader appears for a seccond and nothing happened.
Button remains pressed.

I'm sure there is something in your error log. Did you rebuild your database? What version of Silverstripe do you use?

I use 3.0.55
And yes, i did /dev/build and /dev/build?flush=all too
my log is empty or, rather, there are no today records