Content type export breaks permitted upload file extensions

When I export a custom content type with filefields, all filefields with more than one extension permitted (e.g., PDF, DOC, XLS) get an extra comma inserted between extensions (e.g., PDF,, DOC,, XLS). This in turn prevents any file from being attached and uploaded, with the error message "The selected file FILENAME.PDF could not be uploaded. Only files with the following extensions are allowed: PDF, DOC, XLS".