文档：

The default text to place into an empty field (defaults to null). Note: that this value will be submitted to the server if this field is enabled and configured with a name.

Ext 4.2.2的文档：

The default text to place into an empty field.

Note that normally this value will be submitted to the server if this field is enabled; to prevent this you can set the submitEmptyText option of Ext.form.Basic.submit to false.

Also note that if you use inputType: 'file',emptyText is not supported and should be avoided.

Note that for browsers that support it,setting this property will use the HTML 5 placeholder attribute,and for older browsers that don’t support the HTML 5 placeholder attribute the value will be placed directly into the input element itself as the raw value. This means that older browsers will obfuscate the emptyText value for password input fields.