If you need to put in a rule for the IIS URL Rewrite Module, but need the rule to skip some file-endings and/or targets that are directories or actual files on disk, this is the post for you.
Following some SEO best-practices that tells us to use trailing slashes on our URLs I used the IIS Manager and added the IIS URL Rewrite-module's built-in rule "Append or remove the trailing slash symbol", which creates the following rule:
<rule name="Add trailing slash" stopProcessing="true">
<match url="(.*[^/])$" />
<conditions>
<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
</conditions>
<action type="Redirect" redirectType="Permanent" url="{R:1}/" />
</rule>
This rule takes into account and doesn't apply the rule to files and directories that exists on disk. But there is a big problem with this generic rule.
If you are dynamically serving up files with extensions, then an URL like:
http://website.com/about.html
will become:
http://website.com/about.html/
Adding conditions for specific file-endings
To solve this you can add conditions for certain file-endings, like .html and .aspx:
<conditions>
<!-- ... -->
<add input="{REQUEST_FILENAME}" pattern="(.*?)\.html$" negate="true" />
<add input="{REQUEST_FILENAME}" pattern="(.*?)\.aspx$" negate="true" />
</conditions>
Since the rules above already don't apply for files physically on disk, you don't need to add file-endings like .css, .png or .js.
Update: Match ANY file-ending
If you want to match just any file-ending at all, you use the following pattern:
<conditions>
<!-- ... -->
<add input="{URL}" pattern=".*/[^.]*\.[\d\w]+$" negate="true" />
</conditions>