Handle uploading filenames with special characters

[CGDogan]

Backend part of #640

• A user can input #(.svs and this would be sanitized to .svs which is an invisible system file so sanitized again to svs which turns out to not have an extension thus not usable. The patch sanitizes this to noname.svs
• Send the sanitized filename with "/upload/start", which already received the original filename. This allows user to understand that special chared filenames are not allowed early
• Don't allow a.sVs etc which can't be viewed already. Don't allow for new files only
• File with name '" + filename + "' already exists should also return filename