fix: guess audio mime type for webm extension

[pReya]

Description

Currently all .webm files are inferred to be video files. However, .webm is both an audio AND a video format. In fact it's the default format for most browsers when recording audio-only input via the MediaRecorder API.

When I define my file template like this:

title: Audioaufnahme

accept: audio/webm, audio/ogg, audio/mpeg, audio/wav
fields:
  title:
    label: Titel
    type: text
  created:
    label: Erstellt am
    type: date
    display: DD.MM.YYYY
    time: true
    disabled: true
    step:
      size: 1
      unit: second

I cannot upload an audio file with .webm ending, as Kirby will always complain about the wrong mime type, because it infers video/webm from the .webm file extension.