Creating a file upload field with a translatable title attribute field I have a File field with the title attribute enabled and I'm trying to figure out how to make the title attribute field translatable using entity translations without affecting the actual file field itself. I'd prefer to allow users to upload one file with a title attribute that can be translated.
What I ended up doing was I used hook_i18n_string_info to define a new "Captions" text group, hook_field_widget_form_alter to attach a new process function to the desired upload widgets, and my callback for hook_field_widget_form_alter cloned the title field (into title_fr for me), then also checked to see if #default_value was a string and used i18n_string_translate to update title_fr's default_value.

What I did and wouldn't recommend doing was I put the translation handling in the submit function for my form. In there I constructed my translation "name" using caption:$field_key::$nid::$field_delta and then calling i18n_string_translation_update($name, $french_caption, 'fr', $english_caption).