Set a 'generic' filetype for not supported filetypes

Since `ycmd-mode` is used to check whether completion should work, we have to add some filetype in the request. The filetype cannot be nil or an empty string in the request. 
The real question is what `ycmd` expects, right? If we send "generic" but it doesn't mean anything to `ycmd`, then we haven't solved much. You should check its source/documentation, or ask @valloric what we should pass for `file-types` (if anything) for unsupported filetypes.