The shape of the file you use depends on what shape you would like your finished product to be. A half round file might be best for a semicircular groove, for example.
On the other hand, a square file would be better suited to enlarging a square aperture.
When shaping, double cut files remove material more quickly than single cut files. You might also consider some of the more unusual cuts, such as vixen or millenicut if you are looking for speed.
Suggestions are provided below for which files to use for specific shaping tasks.
Bevelling
Flat, half round (flat face only), hand, handy and mill files
Blades – jimping or serrating
Chequering file, taper saw file and three square files
Car body
Bodifile, flexible file (curved tooth attachment) and vixen files
Chequering
Chequering and three square files
Concave surfaces
Half round, marking, ring and round files
Convex surfaces or rounded corners
Flat, half round (flat face), hand, handy and mill files
Clock or watch parts
Barrette, crossing file and escapement files
Dies
Rifflers
Enlarging round holes
Round files
Enlarging square holes
Equalling, pillar and square files
F-holes (on musical instruments)
Chainsaw files
Fingernails
Hand files
Gears
Barrette and crossing files
Guitar frets
Fret end dressing files
Guitar nut
Round edge joint files
Hinges
Round edge joint files
Hooves
Horse rasps
Ignition points
Ignition point files
Internal angles
Auriform, knife, pippin, slitting and three square files
Jewellery
Barrette, crossing, oval, ring, round and needle files