When using InDesign, you can choose one of two file formats: INDD and IDML. The former is the preferred option because it is a binary file that contains its own cache.
Similarly to previews for other file extensions, you can also upload images and fonts for InDesign projects in the Project editor → General info → Preview files section.
Here are some tips and restrictions you should be aware of:
By default, INDD files do not need to have preview files uploaded since the system draws images from the file cache.
There is a prerequisite to that: you must ensure that the images in an INDD file are linked, otherwise they will not end up in the cache.
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 150 MB.