MIME type error

Hi all, I am having a bit of a problem. I use a form to let the user upload PDF files. However sometimes I get error messages from clients that the MIME type is not support allthough its a valid PDF. What causes the error? I made the following setting my form.md file:

- name: filename
        label: Some label here
        type: file
        multiple: false
        destination: tmp_uploads
        validate:
          required: true
          message: Some error message
        accept:
          - application/pdf
          - application/msword
          - application/vnd.openxmlformats-officedocument.wordprocessingml.documen t

I also accepted word documents to be be uploaded and that works well. It also seems that it's not a general problem as most of the users have no problems uploading a PDF. This error only occurs from time to time.

Just guessing, maybe the MIME type of the file they are uploading is application/x-pdf?

Yeah I did a little research and stumbled over the x-pdf type as well. Will try it with this and test it again. Thanks :)