improve integration with LiipImagineBundle

[lsmith77]

it would be good to have a way to ensure that the file format is included in the paths generated via LiipImagineBundle.

f.e.

{{ /cms/media/foo | imagine_filter('small_filter') }}

might then result in

/media/cache/small_filter/cms/media/foo

however this can cause issues with some web servers and reverse proxies, as they would then set the content type to text/plain.

so we need some solution which would allow forcing the format for the mimetype into the resulting URL.

see also #77 for how to convert the mimetype to an extension