Add support for import of a list of attachments from a multivalued column

Today the API of batch import allows to import files as attachments to imported pages as follows:

• a single attachment named in the value of the mapped column
• multiple attachments, if the value of the mapped column points to a folder.

However, it is not possible to interpret the value from the mapped column as a multivalued list of files and import all the files pointed by the values of this list.

This issue is for adding this feature.