Previous button fails in file upload wizard if a field is required.

I am unable to reproduce the error, as the previous button does not fail for me on the 7.x-2.x branch without the patch. However, the patch itself looks fine to me, so I will mark this rtbc.

I did that. What I did was enabled the file entity and media modules, set up a file field on the article node type, made it required, and then uploaded a file into the field. Maybe I have misinterpreted what this patch is affecting.

Thank you for the clarification. I have tested this more thoroughly now, and there are still some issues that I see. First, after I mark the alt text field of the image file type as required, and then submit an image, the previous buttons works if I fill in the field with a value. That is regardless of whether I have applied the patch or not. If I do not fill in the field, then it fails both with and without the patch, which is what I would expect. Secondly, and this is a more serious issue, but perhaps out of scope for this issue, after I clicked the next button again, the value for the alt text field has been blanked, both with and without the patch applied.

#9 looks good; I've done the same in other multi-step form "wizards". It just needs to incorporate the new step added in #1580820: Provide interfaces for selecting stream wrappers during file upload and converting files between stream wrappers.

Committed #10 to 7.x-2.x.