====== behavior: file ======
Standard file selector. 

===== Elements =====
that have this behavior applied by default: 
  *  ''<input type="file" />'' - inline single line 
  *  ''<widget type="file"></widget>'' 

These elements placed inside ''<form enctype="multipart/form-data">'' will cause content of the selected file to be sent to the server.

===== Attributes =====
that this behavior knows about:
  *  ''filter="filename-filter"'' - defines list of file name filters. ''filename-filter'' is list of filter groups separated by semicolon ';' where each group contains filter name, ':' and comma separated list of file masks. Sample: <code html>
<input name="uploadedfile" type="file" 
       filter="HTML files:*.htm,*.html;Text files:*.txt;All files:*.*" 
       novalue="(select file)"/>
</code>
  *  ''novalue="text"'' - if file was not selected (the input is empty) then it shows text provided by the //novalue// attribute. You can style this state by using '':empty'' CSS selector.
===== Events =====
Together with the standard set of events (mouse, keyboard, focus) //behavior: file// generates:
  *  BUTTON_STATE_CHANGED event - value of the element was changed due to user actions. Posted (asynchronous) event.

===== Value =====

type json::value::V_STRING, reports filename selected.

Note that the value is read-only by code. Only user can set the value from UI. That is made intentionally to prevent arbitrary files to be sent from user's computer.