The favicon extension has been extended to png rather than ico, and mobile also supports png. (Safari, Firefox, Chrome, etc.)
When creating a theme, there is only an ico extension in the source, but please also support png.
The majority of web browsers will prefer .ico over .png
https://caniuse.com/#feat=link-icon-png (hover over note 1). ICO allows the webmaster to better present the icon to be used as long as it has been configured correctly.
You can use https://favicon.io/favicon-converter/ ;(not a reccommendation, checks terms etc before using) to create a suitable .ico file from a PNG if that's all you have available.
I understood well. The option to support png files was also asked by a third-party creator to apply for the theme.
BTW, 5.2 new feature allows you to upload the favicon in the admin panel now, in the file & media manager. Just to let you know. I use The Gimp (free) to make my ico files. We can look at this but we have to research best practices as png files may not be as optimized as ico for this purpose.