Hi @deltan,
Thanks a lot for reaching out to us and using our plugin 
From the screenshot you sent, please ensure that there’s a space and a dot in front of “zip” It should look like this: “image/*, .stl, .zip, application/x-zip-compressed, application/zip, application/x-7z-compressed, .pdf.” Make sure to add a comma and a space after each extension as shown in the example above!
Regarding the STL file, I’ve uploaded a design using our plugin with the same “Accept” configuration, and the upload was successful. I suggest requesting the exact STL file that your user can’t select and sending it to us so we can check if the MIME type is different. This could help us identify the issue 
Feel free to send us an email at support@zeroqode.com with the details mentioned above, including a URL from this forum, and we’ll continue investigating this behavior!
Best regards 
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