Uploading a file with the uploadForm (doesn't matter if it's drag and drop or I select the uploading dialog) from the Media tab works, because it fires the PostAddFile method with the contentTypeAlias parameter set on umbracoAutoSelect.

If I try to upload a non-image file inside a folder it fails, and it automatically treats the file as an image. Even though the item picker menu appears, the contentTypeAlias parameter is already selected as Image.

What could be at fault here?