Problem extracting leap2a zip file on import (Bug #1380434)

For some zip files for some users the mimetype is being interpreted as
"application/x-zip" (note the quotes are part of the string) so this
fails to map to the valid zip types and so the zip file is not
extracted.

So have trimmed the quote off the string

Change-Id: Ia58e3c3aa9a734172543b8c71ecab10a8f313b82
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>