Upon re-edit of an attachment record, the thumbnail link breaks upon submit.
Follow these steps:
1. Create a new record
2. File URL > Redirect to download > Don't scan
3. Upload a thumbnail image
4. Fill in additional fields at will
6. From the File list, re-edit this file
7. Change a field (leave the Thumbnail alone!) eg: change the Category
9. Examine the file record - the thumbnail is displaying the generic icon
10. Edit the file record - the thumbnail file name is listed (eg: thumb_PrePost-Formula-100x55.jpg) but the thumb being displayed is the generic one.
11. Re-upload the thumbnail image again
12. Update to save your changes.
13. The correct thumbnail is being displayed.