Upload more than one File when creating a new Reference
# Feature Request
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## Role-Feature-Reason Template
As a `role` I want `feature` so that `reason`.
## Category according to the [Kano Model](https://en.wikipedia.org/wiki/Kano_model)
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- [ ] must-be <!-- these are the requirements that the customers expect and are taken for granted -->
- [ ] performance <!-- these attributes result in satisfaction when fulfilled and dissatisfaction when not fulfilled -->
- [x] attractive <!-- these attributes provide satisfaction when achieved fully, but do not cause dissatisfaction when not fulfilled -->
## Describe your new feature
Upload multible files by creating a new Filereference
### Current behavior
At the moment one can add multible Files at once to a new Filereference if the Container alrady exists and includes multible files. If one want to add new Files to an exsiting container or into a new container, it is just possible to upload one single file.
### New behavior
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What would you expect the new behavior should look like?
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### Improvement
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What is the benefit and why is it interesting for all users?
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### Screenshots and Mockups
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paste in screenshots or mockups of your new feature
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## Related Issues
- frontend#83 frontend#3
issue