Import images via migration tool

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hello, I'm trying to figure this out for few days. I'm moving large website from old CMS to Concrete5. I'm doing this via Migration Tool (https://github.com/concrete5/addon_migration_tool), but I run into a problem. I'm generating a concrete5-cif XML programatically, but I don't know how to generate IDs for files.

<attributekey handle="sample">
   <value>
      <fID>{ccm:export:file:851521191162:SampleAudio_0.7mb.mp3}</fID>
   </value>
</attributekey>


I'm talking about the "851521191162" part of fID. Where do I get this number? I can't just generate some random number, can I? I tried it before and it caused lot of problems with files not being properly found by the system and so on. What is the right way to do this?