Validator for file extension

e.g. @FileExtension(extensions={"jpg", "png"}, ignoreCase=false)
Annotation without argument's name

e.g.
@FileExtesniton({"jpg", "png"})
doc fix
Validation message for file extension validator
Rename extensions to value for simple annotation

e.g. 
FileExtension({"jpg", "png"})
FileExtension(value = {"jpg", "png"})
Merge pull request #3 from kstefan/file_extension_validator

File extension validator