ie are those below the correct extensions to list?
Yes.
and i assume language string is in general language file?
Yes, but you should not change the default language string and instead install the English language plugin found below and translate from there. Note the mimetypes are substituted in so you'll be hardcoding that language string, which may or may not be shared by other fieldtypes.
www.joomlapolis.com/cb-solutions/languages