Addition for https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us//ApiOverview/Fal/UsingFal/ExamplesFileFolder.html #5576

Current behavior

The section on creating file reference in backend context could use an addition.

Links

https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/ApiOverview/Fal/UsingFal/ExamplesFileFolder.html#fal-using-fal-examples-file-folder-create-reference-backend

TYPO3 versions

Tested in v12

Possible Solution

In the example a file reference was created and added to an existing content element. If you also need to create the record (content element / page / something else) to which the file reference will be connected you can't yet specify the uid_foreign value. This field also doesn't support a "NEWxxxxx" placeholder (the TCA defines the uid_foreign field as number and DataHandler does not check this field for these placeholders).
The solution is to leave the uid_foreign field out of the data array completely (not empty, just remove it completely). When DataHandler processes the image/file field (with TCA type file) it will update the sys_file_reference record and set the uid_foreign field to the uid of the new record.