According to the documentation
TLDR
Files need to have the following
extension:
- .list
- .sources
filename:
- contain letters (a-z and A-Z)
- digits (0-9)
- underscore (_)
- hyphen (-)
- period (.)
SOURCES.LIST.D
The /etc/apt/sources.list.d directory provides a way to add sources.list entries in separate files. Two different file formats are allowed as described in the next two sections. Filenames need to have either the extension .list or .sources depending on the contained format. The filenames may only contain letters (a-z and A-Z), digits (0-9), underscore (_), hyphen (-) and period (.) characters. Otherwise APT will print a notice that it has ignored a file, unless that file matches a pattern in the Dir::Ignore-Files-Silently configuration list - in which case it will be silently ignored.