By creating a new mimetype (say x-gnome/download) based on the file extension
(.part or something like that), and also, by putting at the end of each of these
files a small metadata structure that will contain extra info (% downloaded,
source uri, etc...) This way, by clicking the file the user can easily resume a
canceled/failed download, and also, by creating a thumbnailer the user can be
shown how much download is left and if the download is in progress etc... by the
file icon.

The reasoning for putting the metadata structure in the end is simple. That way
when the download is done it just needs to be renamed and truncated, otherwise
the whole file needs to be 'moved' around. So each time epiphany needs to write
a chunk of data it writes with overwriting the structure, and immidiatly
rewrites the structure. Since the structure should be extremly small it should
not make much of a difference performance wise.