Knowledgebase: iPad/iPhone/iPod touch
Recipient of an attachment forwarded from an iPhone receives an extra file
Posted by - NA - on 30 June 2009 04:34 PM
When you forward an email containing some attachment types from the iPhone/iPod touch/iPad, the recipient receives the attachment, plus an extra .htm attachment. 
These file type attachments have been observed to produce the extra file:
doc, xls, ppt, bmp, txt, gif, jpeg, tif, pdf, vcf, png, tiff