Check transliteration setting before apply transliteration action

Currently, transliteration action will be performed as long as the module is enabled. However, there is a setting variable to control the behavior in transliteration. Sometimes, we need to enable transliteration module to use its functions internally and don't want to apply to all uploaded files.

Just change code from
if (module_exists('transliteration')) {
to
if (module_exists('transliteration') && variable_get('transliteration_file_uploads', TRUE)) {