preview image (before play) not showing

[andiempettJISC]

Describe the bug
when 'preview' is set for a 'streams' in the data.json configuration. currently the preview image does not show at all

example config

        {
        "streams": [{
          "sources": {
            "mp4": [{
              "src": "http://example.com/example.mp4",
              "mimetype": "examplemimetype",
              "res": {
                "w": "1280",
                "h": "720"
              }
            }]
          },
          "preview": "http://example.com/example.mp4"
...
...

To Reproduce
load a page with any working paella player with valid video eg. https://paellaplayer.upv.es/

Actual Behavior
video shown in a stopped state at the beginning of the video. shows the first frame of the video

Expected behaviour
video shown in a stopped state shows the specified 'preview' image

Workaround (if any)
none

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):
all

Smartphone (please complete the following information):
all

Additional context
tested a previous version of paella 5.3.x and previews is working OK

Thank you for contributing!