Having been using the plugin Media File Renamer for quite some time with great success, it seems like with one of the latest updates, the thumbnails are not generated anymore.
After adding a picture to a post, and saving the post, the image is renamed, but is not visible in the post anymore, nor the thumbnails in the media gallery. However, when editing the image in the post, and trying to edit the original image, it displays correctly, which is quite strange.
After disabling the plugin, deleting the pictures, and uploading them again, same problem : thumbnails do not display.
Two solutions are available : either install a plugin that will allow to regenerate the thumbnails like Regenerate Thumbnails (did not work in my case), or rename the images before uploading them again, solution which worked for me.
After renaming the files in Windows explorer, to have different name than the previous ones
And uploading again the images in the post, they displayed again correctly, both in media gallery
And in the image options
Another solution is to install a plugin that will allow to regenerate thumbnails
Then, in the media gallery, find the image to update, and click on regenerate thumbnails
By default, only the missing ones will be generated – however, it is possible to regenerate all of them