If you wish to have a file field that can be used optionally, use the attribute allowEmptyValue="true" inside of the nested spec element:
<field type="file" variable="the.file"> <spec txt="" size="25" allowEmptyValue="true" set=""/> </field>
This supports a <description> element that can be used to provide information about the purpose of the file field.
See Fields documentation for more detail.
<field type="file" variable="password.file"> <description id="password.file.info" txt="Where would you like to place your password file?/> <spec size="25"/> </field>
Note: Mixing file fields with other fields that have text in front of them can lead to formatting (layout) issues. Placing these types of elements on different panels can provide a much better user experience.