Allowing the .zip files to be detected by file extension (Bug #1302251)

A number of filetypes are being detected as a zip file, due to them
having compression.

To sort this out we will let the file extension be checked first then
fall back to checking magic.mgc.

Change-Id: Iab6ddbc17af4018cf381c4949172804acff49483
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Allowing the .zip files to be detected by file extension (Bug #1302251)" into 1.6_STABLE