Tell me for which of the following file types a SEO expert needs to provide an alternative textual description?

Submitted by: Muhammad
(swf. - gif.) Since .html, .pdf and .doc are text file formats and spiders can read their contents directly, it is pointless to provide an alternative textual description. On the contrary, .swf and image file types, like .gif and .jpg, need alternative textual descriptions because spiders cannot read their contents directly.
Submitted by: Muhammad

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