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Fonts and images must be in one zipped folder.
You should check if files are zipped correctly, and their file extension is the same as it was before the zipping process if, for example, you had “.TIF", but its name in a ZIP file was ".tif", the files would not load.
Preview images must be in one of the four file formats that are valid in XTM Cloud: JPEG, PNG, SVG, GIF.
There is a UI and API limit to file size which is currently 700 150 MB.