File attribute - HTML input mode swipes the current value when updating a record

[ahukkanen]

I find this issue with Express (not sure if it applies everywhere for file attributes):

I have an existing record with a file assigned to it through the file attribute (using HTML input method). When I update one of these records, the updating process swipes out the current value if the field was left empty.

This makes it very hard to keep these records up to date as you would have to upload the files again every time you do an update.

[ounziw]

I'm not tested 8.2.1 or 8.3rc.

I published a composer image upload function skipping file manger for 5.7.5.x
https://gist.github.com/ounziw/56e2b4f2602cafa29f84020768e44b37
Here, I define a hidden form for "value", so that it stores an ID for existing file.

I hope this code will help fixing the code for 8.2.1

[mlocati]

See #7843