In some cases the automatically-generated thumbnails created by Asset Bank for certain file types are not satisfactory. For example, if the source file contains transparency on a white background then the resulting JPEG thumbnail might look all white.
The method to get around this involves changing a record in the database that explicitly allows 'thumbnail files' to be uploaded for the given file extension.
The statement needed is:
UPDATE FileFormat SET CanUploadThumbnail = 1 WHERE FileExtension = 'jpg';
Replace jpg with the file extension of your choice. Once you have uploaded an Asset you can edit it to replace the thumbnail.
N.B. this functionality can only be used if Asset Bank's substitute files feature (AKA working files) is turned off, i.e. in order to use this custom thumbnail feature you must have the following in ApplicationSettings.properties: