Handle åäö in filenames (Invoice Generator front)

[IiroP]

If I upload file testiåäö.png, it is passed as testiåäö.png (in NextJS before sending to actual Invoice Generator).

Original issue in backend repository: Tietokilta/laskugeneraattori#33

Maybe relevant:

• https://stackoverflow.com/questions/93551/how-to-encode-the-filename-parameter-of-content-disposition-header-in-http
• https://datatracker.ietf.org/doc/html/rfc7578#section-5.1.2