fix images detection issues for file_upload types

[Laith-jalal]

This PR fixes an issue for the FileUpload type especially for the :image? instance_option,
some file uploading Gems add a query string part to the file/image URL, paperclip for example adds timestamp value to the file's URL and in the current implementation having query string will result in NOT considering the file as an image even when it's an image

[coveralls]

coverage: 95.866% (+0.001%) from 95.865% when pulling d209b87 on Laith-jalal:fix_file_upload_images_detection_issue into 647347c on railsadminteam:master.