The documentation for file_unmanaged_prepare() claims that the $source parameter is "A string specifying the filepath or URI of the source file."
Unfortunately, the first thing it does is call file_exists($source) which only accepts a path as input. So a URI fails that check.
I ran into this when trying to use file_unmanaged_copy() which calls this function.
Needs work
11.0 🔥
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The first step should be to update the documentation to clarify the URI only supports a local streamwrapper. We can then create a follow up to discuss adding remote streams.
Also file_unmanaged_prepare() was deprecated in drupal 8.7.0 and replaced with \Drupal\Core\File\FileSystem::prepareDestination() so we need an IS update.