I had the same issue with svg. Media library must treat svg as an image, so the preview will work
You can add this key to web,config:
<add key="CMSImageExtensions" value="svg;bmp;gif;ico;png;wmf;jpg;jpeg;tiff;tif" />
Or you can modified the code where the rendereing happens in \CMSModules\MediaLibrary\Controls\MediaLibrary\MediaFileEdit.ascx.cs
and there around line 430 in the private void SetupFile()
you need to change
if (ImageHelper.IsImage(FileInfo.FileExtension)
to
if (ImageHelper.IsImage(FileInfo.FileExtension) || FileInfo.FileExtension == ".svg")
P.S. File The App_Data/mimetypes.txt file used to register custom media types was removed in Kentico 10. Read more here