You need to join to the file table twice. For that reason you need to assign the table two different aliases so there is no ambiguity when referencing it.
select
category.id,
bannerfile.[path] as bannerimagepath,
thumbfile.[path] as thumbimagepath
from
[category]
left join [file] as bannerfile on category.bannerid = bannerfile.id
left join [file] as thumbfile on category.thumbid = thumbfile.id