Using creator.onUploadFile to handle image uploads in Theme Editor - Introduce an option which would indicate a sender (e.g., a survey's background or header background property)

[JaneSjs]

T16961 - Detecting sender when file uploaded in theme https://surveyjs.answerdesk.io/internal/ticket/details/T16961

User inquiry: update the creator.onUploadFile event and add information about whether an image is being uploaded for the header background or a survey background image.