fixing bug #4685 Vignette : non prise en compte des extensions de fichier en majuscule
bug #4685 :  Merge branch 'bug-4685' of github.com:CecileBONIN/Silverpeas-Components into bug-4685
bug #4685 :  The bug fix introduced duplicate codes instead of improving the FileUtil#isImage method. So remove this duplication and kept the call of FileUtil#isImage to check the file is well an image