Fixed Image upload field processing on volunteer signup form

[pradpnayak]

For #518 and https://civicrm.stackexchange.com/questions/31933/image-file-upload-issue-on-civivolunteer-public-form

[ginkgomzd]

I replied on the SE thread.
This PR looks fine, but I'd like to understand what is going on.
Right now, this is looking to be caused by a change in core, so I'd like to know what version the change was introduced. In particular, is this fix backwards compatible? I can go check that right now...
After trying the patch, and looking more closely at the profile, I am leaning towards this being an instance configuration issue (or permissions? or caches?). I see that the profile should automatically adapt to an upload-type submit with the presence of a file-field.