It is quite common to find a gusset of sorts in the corner(bend region) of sheet metal parts to stiffen the part. To accomplish this, we'll create a custom form tool (or copy the one attached) to define the shape of the gusset. We'll then apply the form tool to the part, orient and locate the form tools sketch and close the dialog. The result is a formed gusset across the bend of a sheet metal part. This "looks" good but is not flatten-able. To get the flat blank/pattern you'll need to first suppress this form tool.
Note: this part is 12ga (.1046) material and this also appears in the form tool part, these need to match.