Took me a while to track this down tonight. I thot that the HtmlHelper.Form would default to a POST method and so didn't explicitly declare it as such. Therefore, when I did this (using Brail):
${HtmlHelper.Form('new.rails',{'enctype':'multipart/form-data'})}
${HtmlHelper.LabelFor('photo','Title:')}
<br />
${HtmlHelper.InputFile('photo')}
<br />
${HtmlHelper.EndForm()}
I kept getting complaints from the Controller saying it couldn't convert 'String' into an 'HttpPostedFile' object.
It turns out that I needed to include a 'method="post"' explicitly in my form helper. I'm sure its a rookie mistake but still not much info to figure it out. Now it is this:
${HtmlHelper.Form('new.rails',{'method':'post','enctype':'multipart/form-data'})}
${HtmlHelper.LabelFor('photo','Title:')}
<br />
${HtmlHelper.InputFile('photo')}
<br />
${HtmlHelper.EndForm()}
And it works...