Currently you can only map an Item property back to a file if the property physically exists within the file. File properties can be created that are not actually mapped back to the file metadata. This is used for many different workflows within Vault. Vault does not allow these unmapped properties to report back up to Items.
For example: We include 3rd party PDF drawings in Vault that we manually edit the properties, like description, revision, etc. We cannot map these properties to the associated Item since they physically do not exist in the PDF file.
