Since a folder could be the container for a file, the problem is
obviously caused by that famous rhetorical device, synecdoche ...

"A figure or trope by which a part of a thing is put for the whole
(as, fifty sail for fifty ships), or the whole for a part (as, the
smiling year for spring), the species for the genus (as, cutthroat
for assassin), the genus for the species (as, a creature for a man),
the name of the material for the thing made, etc."

Or, in this case, the container for the thing contained.

Marion Matters
Minnesota Legislative Reference Library
645 State Office Building, St. Paul, MN 55155
Phone: 612/297-8891 Fax: 612/296-9731
Email: [log in to unmask]
Library's web site: http://www.leg.state.mn.us/lrl/lrl.htm