If you rename a file, the thumbnail will disappear. Renaming the file back to
the original name will make the thumbnail reappear.
You can't work around it by deleting the file and resubmitting with the same
thumbnail, as it will give you a 'file already exists' error for the thumbnail
(unless you rename the file before uploading). You also can't just upload a new
thumbnail, as the edit upload feature doesn't allow for that.

To recreate (in backend):
1) upload a file and a thumbnail
2) submit
3) open upload and rename it
4) submit
(frontend will no longer show the thumbnail)

To work around:
-rename upload back to the original name
OR
-delete upload, then go into file manager and delete the thumbnail, and recreate
it all.

Expected behaviour:
-uploaded thumbnail should still work even after renaming upload.