After uploading documents, audio files, or images under the Attachments section, you can preview some file types using the preview tool (the one that looks like a magnifying glass). A lightbox appears and allows you to see the content of the file without having to download it.
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Below are the acceptable file types that can be previewed:

Images
png, jpeg, jpg,
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​Documents
txt, xls, doc, docx, xlsx, pdf, rtf
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​Audio/Video
mp3, wav, ogg,
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Note: You can still upload other file types (see the complete list here), you will just have other file types to see their contents.