In version 6.3, it is not possible to use file as an attribut of a business object (check here for the list of possible data types). But you can save a url to your file/picture in a text attribute.
Or you can use documents on the process level.
Bonitasoft empowers development teams with Bonita, the open-source and extensible platform to solve the most demanding process automation use cases. The Bonita platform accelerates delivery of complex applications with clear separation between capabilities for visual programming and for coding. Bonita integrates with existing solutions, orchestrates heterogeneous systems, and provides deep visibility into processes across the organization.