Add opus to the list of allowed extensions in the uploader

here is the line: https://code.eliotberriot.com/funkwhale/funkwhale/blob/develop/front/src/components/library/FileUpload.vue#L69

We can probably use the extensions attribute instead of filtering by mimetype, cf https://lian-yue.github.io/vue-upload-component/#/en/documents#options-props-extensions