If you receive an e-mail with an attachment, you can tap on the attachment to view it. If it won't open, the file is corrupted or it is a type that is not supported by the iPhone, iPad or iPod touch.
The following formats are supported: Microsoft Word (.doc), Microsoft Word XML (.docx), Web page (.htm, .html), Keynote (.key), Numbers (.numbers), Pages (.pages), Adobe Acrobat preview (.pdf), Microsoft PowerPoint (.ppt), Microsoft PowerPoint XML (.pptx), Plain text (.txt), Contact information (.vcf), Microsoft Excel (.xls), Microsoft Excel XML (.xlsx).
Note also that the file attachment must have the correct file extension or you won't be able to view it. So, for example, if someone sends you a Word document without the .doc extension, you won't be able to view it.