Steps to reproduce the issue

1. Create an image folder like "/media/photo/2019" outside joomla's "images" folder;
2. Create a symbolic link of a subfolder "media" in "images" folder for the "/media" folder;
   (so that images/media/... symbolic folder name can access to the physical folder "/media/...")
3. Configure TinyMCE's plugin setting "Images Directory" to have "media/photo/2019";
4. Edit an article with TinyMCE editor, and try drag-and-drop an image file into the edit window.

Expected result

The drag-and-dropped image should be shown in the article's edit window after the image file was uploaded to the subfolder "/media/photo/2019/".

Actual result

The image file was not uploaded, and "Unable to upload file.: /jmlundefined" error message was popped up.

System information (as much as possible)

Joomla 3.9.6. PHP 7.2.18

Additional comments

This problem happened since Joomla 3.9.5, and was not fixed in 3.9.6.
Only a symbolic linked path for Images Directory of TinyMCE seems to show the problem, but no problem with a physical path setting for Images Directory setting of TinyMCE plugin.
A symbolic directory link had worked fine with image drag-and-drop in TinyMCE editor before upgrading to 3.9.5.