Task Import Wizard: Adding files is validated only against the system default BCONF

When adding workfiles in the Import Wizard, the file-extension of the added file is validated against the list of extensions from the system-default bconf. This is of course problematic and not correct.

There are two possible strategies to fix that:

a) Validating against the currently selected bconf:

• needs all extensions as data in the frontend. That will need this data to be saved in LEK-okapi_bconf as well as parsing all BCONFs on-the-fly is inappropriate
• When changing the bconf and files are already added, these must be invalidated
• rather complex coding

b) Validating against the "SUM of all provided extensions"

• This would still let users upload files not sopported by the selected bconf
• Solves the Problem with added extensions in a bconf
• Backend needs to gather this sum. This also requires adding the Extensions to the DB
• Frontend need not much change: The selector/dropdown of bconfs should be enhanced by adding the custom extensions of a bconf behind the name, eg: "My special Figma BCONF (.figma, .xyz)"