Original PR description

This commit fixes a bug introduced when migrating
from jquery deferred to native JS Promises (ab56e63).

Steps to reproduce:
1. Go to the Expense app
2. Create a new expense
3. Click on "Attach Document" and upload a file
=> crash and infinite loading spinner

After loading files, the `attach_document` widget is
supposed to call a callback action with attachment ids
as argument.

Currently, attachement ids are not correclty retrieved.
They are retrieved via the `arguments` object but since ab56e63
`arguments` refers to the arguments of the Promise callback
function and no longer to the handler function `_onFileLoaded`
arguments.


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