Rename the filename of File field in a Channel Form.

• Copy the /system/expressionengine/third_party/channel_form_file_rename/ folder to your /system/expressionengine/third_party/ folder

Add a field to your Channel Field form, whose name is your File field name with _rename at the end.

{exp:channel:form channel="site" return="my/form/ENTRY_ID"}
	<input type="hidden" name="my_file_field_rename" value="{exp:guid:uniqid}">
{/exp:channel:form}

CREDIT: Based on a modified copy of an addon by Rob Sanchez: http://github.com/rsanchez/channel_form_file_add_prefix