Project version reports no primary file, yet UI designates one as primary

[itzg]

Describe the bug

When retrieving a version of a project with https://api.modrinth.com/v2/project/698NGGtb/version?loaders=%5B%22paper%22%5D&game_versions=%5B%221.19.3%22%5D

Then no files are designated as primary in the response:

    "files": [
      {
        "hashes": {
          "sha512": "8040d1bdb4cd9c5a0976ab2b41005b8405ad0e56894e84b5192541c6338c4bce404008808f78085e5020081c7f04876275a6014534dbd5835d63c7851195bac5",
          "sha1": "80e080ca558ab366da36ea87b9baa97a7edb08a7"
        },
        "url": "https://cdn.modrinth.com/data/698NGGtb/versions/AlzUrzCs/all-plugins.zip",
        "filename": "all-plugins.zip",
        "primary": false,
        "size": 3843079,
        "file_type": null
      },
      {
        "hashes": {
          "sha512": "a83a63e83dfc375d23e7ba57e77fdc4a91a9de5d46b545b3b79f708c4107fbdade5291eabb31acd80a4f0716589693320f2f714a446e62273c0dd2929fdc0797",
          "sha1": "76524335d6fa0f911fcd801cc12c9506e58a14a7"
        },
        "url": "https://cdn.modrinth.com/data/698NGGtb/versions/AlzUrzCs/vane-plexmap-1.10.3.jar",
        "filename": "vane-plexmap-1.10.3.jar",
        "primary": false,
        "size": 13395,
        "file_type": null
      },
      {
        "hashes": {
          "sha1": "23b68b8d2984eac046966c138e5393df8bcbf013",
          "sha512": "1336036172d4f4a4d10bc09cfca2bb4bc58922bf86ad03931ac8bc212deee1bd23ec69b2116d71c1c2c2da0b4d2d857696a6e379436b9b1ff3f7337a69940846"
        },
        "url": "https://cdn.modrinth.com/data/698NGGtb/versions/AlzUrzCs/vane-velocity-1.10.3.jar",
        "filename": "vane-velocity-1.10.3.jar",
        "primary": false,
        "size": 171997,
        "file_type": null
      },
      {
        "hashes": {
          "sha1": "bc83207bc87c541b03d45310cc0fb747d0d06b82",
          "sha512": "f91187ae8406a3c1ad41cc21d585c8bb9185942e3d946e4797ebbe0321f3d75d3d113f775827a7080203811c090641a6bd3c1af5db9fee18dc955731c73ea203"
        },
        "url": "https://cdn.modrinth.com/data/698NGGtb/versions/AlzUrzCs/vane-waterfall-1.10.3.jar",
        "filename": "vane-waterfall-1.10.3.jar",
        "primary": false,
        "size": 406122,
        "file_type": null
      }
    ],

however, the UI at https://modrinth.com/plugin/vane/version/v1.10.3 marks the first one primary

Is there an implied behavior that the first file is primary when none are declared primary?

Steps to reproduce

See description

Expected behavior

At least one item in files to have primary as true.

Additional context

Originally reported here itzg/docker-minecraft-server#1973 (comment)

[Geometrically]

Yes. If there is no file with primary set to true the primary file is the first file in the array.

[itzg]

Is that documented somewhere?

[triphora]

It is now: modrinth/docs@b09d9fc

[itzg]

Awesome, thanks @triphora ! Got the new logic incorporated earlier today, so brings it all together nicely.