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Showing any type of the file is a function, very specific to the browser and I don't think you can do much about it.
For example, many of the browsers will show text files, xml files, pdf files within browser window. This all depends on browser plug-ins.
Furthermore, please be also aware that an end user can turn of showing contents within the browser window by changing the preferences. In this case, the file will prompted for download to end user.