Is it possible to get the internal id of a file in a 'Document' type field? By just selecting it, I get the error

Search error occurred: Field '_fieldid_' for record '_recordid_' was not found. Reason: UNSUITABLE - Unsupported data type 'DOCUMENT'

I don't think any of the Built-in Functions would help here, and can't find anything online or in SA

Curious about this myself. So far, I have concluded that Document fields simply are inaccessible via SuiteQL, though I can’t find that fact documented anywhere in SuiteAnswers. For cases where I need them, I have fallen back to using saved searches 😞

I was thinking I'd make a hidden integer field that sourced the document's internal id in the filecabinet. No luck figuring out how so far...

you could have a before submit function execute that sets a value of an integer field anytime the document field value is changed. Then you could query the integer field instead of the document field