Restore form attributes after successful upload in io-upload-iframe

Fixes #2533186
Merge branch 'dev-master' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x
Merge remote-tracking branch 'upstream/dev-3.x' into dev-3.x