cubicweb-expense #2990592 attach files to expenses [validation pending]

in order for example to be able to attach scanned invoices.

prioritynormal
typeenhancement
done in0.7.0
load0.200
load left0.000
closed by#3ee88814c42f attach files to expenses (e.g. scans) (closes #2990592)
patchattach files to expenses (e.g. scans) [applied]