Postie cannot load any PDFs

Hi there,
always getting this error message:

….2024.pdf has an unsupported MIME type application and was not added.
Supported types:
Array
(
)

disabling all other plugins, adding pdf as supported file type at postie plugin, changing filenames etc. didn’t help.
Postie does upload any other image or video without problems though.
Uploading pdf files manually via media manager works just fine too.

The page I need help with: [log in to see the link]