fix csv upload: accept correct content type

[nk-coding]

🎫 Description

Fixes the bug that the csv import loads forever.

• replaces hardcoded .csv in accept with the content types
• remove check for content type afterwards as it is now already checked by the upload itself (therefore it can never be an invalid content type), and if the validation fails it would only cause the bug of an infinite load
• changes the content type for the csv import to text/csv, as application/vnd.ms-excel seems to be .xls and not .csv

🔒 Closes

• Fixes Import user from csv #1011