Grav V. 1.7.0 - rc.12: Admin Panel Upload MIME type not accepted ERROR

Hello,

after updating to the latest Grav Version 1.7.0-rc.12 and Plugin Admin Panel to v1.10.0-rc.12 i am not able to upload files with the Admin Panel. Before this realease all worked properly. The Configuration - Media is set up correctly.

When i upload i.e. a PDF file i get this error:
Save Failed: The MIME type “application/pdf” for the file “test.pdf” is not an accepted.

I am using Grav on my website: www.unsere-schule.org with the following versions:

Grav Version 1.7.0-rc.12
Admin Panel v1.10.0-rc.12

Thank you in advance