It seems that I can only filter files to png/gif/jpg and text. Why it is not possible to filter image/svg or text/bxf2? (bxf2 is an own xml format /ending)
There are two levels of filtering
onBeforeFileAdd event can be used to define any kind of js logic, https://docs.webix.com/api__ui.uploader_onbeforefileadd_event.html
accept configuration property, uses native browser functionality
So, in the case of accept we are limited by the browser's functionality. If you want a full control, use onBeforeFileAdd ( user will be able to select file, but it will not be added to the upload list )
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