thumbnail is always broken

no matter what i try, i keep getting a missing-file icon for the thumbnail. the broken thumbnail is a img tag that actually links to a .swf file in my files folder (iow the file does exist but is not an img). oddly, the "custom thumbnail" fieldset is available regardless of whether i have custom thumbnails turned on for this field. but in either case (custom thumbs turned on or not), whenever i upload a custom thumbnail and save the node, when i re-edit the node the field is blank again. the custom thumbnail does exist in the files directory and has an entry in the files table in the database, so i'm not sure why it isn't showing up.