If you have a repeat tag and want to get the filetype from the first element you can't access the first element in a from_param type.
That commit creates some artifical parameter called 'first' that you can use as value in param_attribute. For example:

	<option type="from_param" name="singlePaired.mate_list" param_attribute="first.input_mate1.ext" />

I use that to retrive the filetype of paired end reads that are inserted in a repeat tag.
Assuming that every mate pair is from the same type as the first element.