PNG file is considered mp3

[lorenzograndi4]

While testing the v2 POST /downloads endpoint for mobile; @renssies saw one of the 8 PNG files was considered audio/mp3 in the output. URL with the whole transfer here. See original file below. We do have permission to use the attached file as a test case.

See what the output was like for this file in the response body, compared to another PNG file in the same response:

    {
      "id": "0b9a11e1bb8fbeafb7a76cb5763fbeaa20180815082345",
      "name": "Simulator Screen Shot - iPhone SE - 2018-08-14 at 13.25.53.png",
      "size": 456833,
      "display_width_px": null,
      "display_height_px": null,
      "preview_url": null,
      "format": "mp3",
      "nature": "audio"
    },
    {
      "id": "0e3fba318ddc88edcc1082de4414b1b620180815082345",
      "name": "Screen Shot 2018-08-14 at 13.25.03 (2).png",
      "size": 991783,
      "display_width_px": 3360,
      "display_height_px": 2100,
      "preview_url": <REDACTED>,
      "format": "png",
      "nature": "image"
    },

[lorenzograndi4]

Closed in #132