The membership_contract custom field in membership_general custom group can hold
a contract reference. This contract reference is matched to a file located in:
sites/default/files/civicrm/custom/contracts/{reference}.tif
If the file does not exist, it will not be available for download and the contract reference will not be shown as a link.
You must create a directory or symlink in CiviCRM customFilesUploadDir
"contracts". eg. sites/default/files/civicrm/custom/contracts for Drupal
environments.