Suggest correct file extension when cropping pdf, djvu and tiff

[bawolff]

Previously this was suggesting a .jpg extension for multipage files and keeping the same extension for everything else.

However, single page .pdf and .djvu should still have the .jpg extension. Multipage .tiff files should keep .tiff and not be .jpg

This was causing upload failures unless the user noticed and changes the extension.

Some examples of affected files: File:adrenaline_ampoule.pdf or File:1898_circa_Willy_Hoehl_Gruss_vom_Nordstädter_Gesellschaftshaus_Gustav_Fischer,_Oberstraße_7_und_8,_Adressseite.tif