I was using the UploadField like this

        $imageField =  new UploadField('Image','Choose Image');
        $imageField->setFolderName('Uploads/promotionalbanners');
        $imageField->setRecord($this);

Now in 3.4 it has to be:

        $imageField =  new UploadField('Image','Choose Image');
        $imageField->setDisplayFolderName('Uploads/promotionalbanners');  
          //set where a new one is chosen from in tree dropdown .
        $imageField->setFolderName('Uploads/promotionalbanners'); 
          //set where images are actually uploaded to
        $imageField->setRecord($this);

Prior to 3.4 when one tried to choose "From File" or "Choose Another File" the resultant dialogue box would be open in the right place, in this case Uploads/promotionalbanners. However when a new file is dropped in there it is correctly saved to Uploads/promotionalbanners so the code is partly working

I have been using this code since 3.1 - years and it has always worked perfectly until now. What has happened?

On 30 August I found that one has to set ones field like this:

<code> $imageField->setDisplayFolderName('Uploads/promotionalbanners');</code>

Research then showed that Chillu has requested a pull ages ago with this in it. https://github.com/silverstripe/silverstripe-framework/pull/2064
This only seems to have reared its ugly head in 3.4 (maybe 3.3 too)

Any news here?
I have the same problem. What was changed here?

Fixed it. See first post