Hi Cate Detheridge,
WMF and EMF files are vector based, so there really is no image from which to make a thumbnail from. A vector based image is nothing more than a mathematical set of instructions to remake some image, similar to MIDI files are on the audio side don't contain any waveforms.
You can of course convert these EMF (Enhanced Meta File) files to a raster based graphic file that should generate a thumbnail.
Windows 7 does not generate the thumbnails for these vector based files.
However there are several non- Microsoft programs which would convert and generate the thumbnails for these filetypes, you may use your favorite search engine and download these programs and check.
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Thanks and Regards:
Swathi B - Microsoft Support. Visit our Microsoft Answers Feedback Forum and let us know what you think.