UploadFileHelper - change api to support multiple files

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #39
| License       | MIT
| Doc PR        | n/a

This changes the api of the UploadFileHelper to support uploading multiple files in future. We also prevent BC breaks if we want to add the logic for that later.

Furthermore I added an app test page to demonstrate and test the upload and download of a file.

**TODO to merge:**
- [x] add unit test for UploadFileHelper
- [x] add web test case to upload and download a file