Today I have discovered that there's a problem with uploading via Tumblr links - both direct links to illustrations, and links to posts, *and* permalinks. The error code I got with the first two was:
Upload failed: Failed to replace upload_media_assets because one or more of the new records could not be saved..
While the permalink got me:
Upload failed: File is not an image or video.
I have a few theories why it may be:
1. Nowadays, Tumblr sometimes converts .png to .pnj, and .gif to .gifv - reflected in the file's URL.
2. When viewing a file through a direct link, like this one (Tumblr post link here), only a blue background is shown if not logged in - the file only appears properly when logged into Tumblr, for some reason. It may be a copyright protection, like on Pixiv, maybe?
3. The permalinks are more of a redirect to a Tumblr post.