Split extension from database file format

Currently pootle use the file extension in order to identify which format use.
- The issue is that an extension(.properties) can be used by multiple format (Java properties / Java UTF-8 properties / GWT properties)
- We need to set the source and the target language of the store before handling it. Indeed it can be usefull for the converter to know for which target language the call is made.
For example, in order to export a .properties file from pootle we need to know which language to use in order to know which form is each elements of the multistring provided for the translation

(Need proper testing, i only made it works in my env)