The drag and drop feature does not work within Jim2Cloud.
To add a document to a contract, view the contract, then drag and drop the file (from the external drive/network) directly onto it.
A document added in this way would normally be flagged as Hidden, meaning that it won't be displayed in a document list, unless the Show Hidden icon is selected.
With the contract in View or Edit mode, select the Add Document from Clipboard icon in the ribbon:
Once added, the document will be accessible via that contract’s Documents tab (in the footer). A number will be displayed on the tab to show the number of documents linked to the contract. That number will increase as more documents are added to the contract.