Improved filename verification of illegal characters when uploading a file

### Description

We had one occurence where a user somehow was able to upload a file with a colon (:) as part of the filename. We accepted this and stored the file in blobstore, but this caused issues further down the line when the service owner tried to download and process the file.

### Additional Information

_No response_

### Tasks

_No response_

### Acceptance Criterias

- [ ] Given that a user tries to upload a file with illegal characters (:), when the app backend receives the request, then the app backend should reject the file and given the frontend a propper response identifying the problem

Start by tracing the backend upload request handling described in the issue and identify where filenames are accepted and where validation errors are returned to the frontend. Done means uploads containing the specified illegal character are rejected and the frontend receives a response that identifies the filename problem.

Written by the indexing model from the issue text.