Only allow upload of certain document types

[annkatrinkuessner]

As
PO
I want to
ensure that applicants only can upload PDFs for their "Einverständniserklärung" and have them checked for viruses
In order to
ensure that our system is save.

Acceptance Criteria:

• show validation errors to users
• provide a little guide on how to generate a PDF
• show warnings to admins of the platform if a file might contain a virus
• move infected files to sandbox

Additional Idea:

• if a user uploads multiple pages (e.g. 3 times one page) make one 3 pages PDF out of them

Please look out for gems that might help to implement this...

Partly linked to #555

[annkatrinkuessner]

for admins of the portal it still should be possible to upload other file types.

[Baschdl]

References: #100