Was past my bed time last night. I just created a file field on the front end of a test site and I added your filter to allow uploading of .pdo files. The field and the upload worked perfectly. This is a site that is the current version of WP and the only plugin enabled on the site is ACF.
The last thing that I want to be sure of is that this is a “File” field as you mentioned in the OP. The errors seem to indicate that it’s an image field or for some reason that something is trying to treat the .pdo as an image file. Unless you have some other plugin that recognizes .pdo as image files, .pdo files are not image files.
Beyond that, there must be something going on with one of the your other plugins. You will need to figure out which one is causing the conflict.
