Students cannot export assignment feedback to portfolio

To reproduce:

1. Student uploads document to file upload assignment
2. Teacher downloads, grades and comments on document
3. Teacher uploads document as assignment feedback
4. Student clicks "export to portfolio" for the feedback document
5. Student gets error:

 
Sorry, the requested file could not be found

 [More information about this error

Debug info:

Error code: filenotfound

Stack trace:

• line 9499 of /mod/assign/locallib.php: portfolio_caller_exception thrown
• line 185 of /portfolio/add.php: call to assign_portfolio_caller->load_data()

Student can, however, still download the file by clicking on it. They just can't export to portfolio.