Endchan doesn't offer a "strip filenames" option for file uploads, unlike some other chans which rename the file to its hash. Such functionality could be nice on Endchan, too. If Endchan doesn't support it, it could be possible to predict the filename without uploading it to Endchan. Looks like the filenames are generated with base64 and md5 from the file data:
https://gitgud.io/InfinityNow/LynxChan/-/blob/master/src/be/engine/apiOps.js?ref_type=heads#L160
    var toPush = {

      title : file.name,

      md5 : crypto.createHash('md5').update(content, 'base64').digest('hex'),

      size : stats.size,

      mime : mime,

      spoiler : file.spoiler,

      pathInDisk : location

    };