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. 
AFAIK `ycmd` uses the filetype also for grouping when using the identifier-based engine. i.e. to get the identifiers from bufffers with the same filetype.