Try this
$.getURLForImage(fileid=$.content('headingFile'),size='large')
or
$.getURLForImage(fileid=$.content('headingFile'),width=600,height=300)
Or if it's not an image and you just want to download it
$.getURLForFile(fileid=$.content('headingFile'),method='inline')
or
$.getURLForFile(fileid=$.content('headingFile'),method='attachment')