[BUGFIX] Slighly refactor the getAllowedExtensions to support FE

Change-Id: I2d4df1feac34a945619ef0f06f03dbb2b8dd833f
Reviewed-on: http://review.typo3.org/32576
Reviewed-by: Fabien Udriot <fabien.udriot@ecodev.ch>
Tested-by: Fabien Udriot <fabien.udriot@ecodev.ch>
[BUGFIX] Fix call to mistyped method name

Change-Id: Ic24a23a21635791fcd00ea78a76384bedbd07e6d
Reviewed-on: http://review.typo3.org/32577
Reviewed-by: Fabien Udriot <fabien.udriot@ecodev.ch>
Tested-by: Fabien Udriot <fabien.udriot@ecodev.ch>