there is sadly no source-type: binary or something similar and the files you try to download seem to be raw binary files …
source-type
Type: enum
Used when the type of source entry cannot be detected.
Can be one of the following: [bzr|deb|git|hg|local|mercurial|rpm|subversion|svn|tar|zip|7z]
(from https://snapcraft.io/docs/snapcraft-parts-metadata)
this is similar to: