If you are uploading a file and use the 'Select file for uploading' and click on the Browse button it will upload a file you select from your local device - this could be called a 'Download with an internal file'
jD forms the link appropriately.
If you are using that form of link in the External or Mirror links then it will not work!
An external link would, as you noted, be an explicilt link to a Google drive or similar
There are three 'internal' link possibilities:
Select File for uploading or Select for Existing Download or Select File from Root area.For these jD generates the link automatically.
Ignoring the mirror sites there is only one 'external link'.
For this you need to specify the actual remote location address of the downloadable file.
The Mirror sites are also ana type of Extrenal link but generally used to give alternatives for the same downloadable file.