[Files.app] Clean-up the code around checking file-extension

- Fix inconsistent expressions of file-extension (with-dot or without-dot)
- Use PathUtil methods instead of self-implementation

BUG=none
TEST=manually

Review URL: https://codereview.chromium.org/101183002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238950 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238950
Update Valgrind excludes (remove a test that no longer exists).

BUG=280495
TBR=dominikg@chromium.org

Review URL: https://codereview.chromium.org/105613006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238951 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238951