您需要将标题设置为multipart/form-data

$request = Http::withHeaders(
 [
  'Accept' => 'multipart/form-data', 
 ]
)

问题可能出在Storage::mimeType。它返回image/jpeg。您的文件名将变为firstupload.image/jpeg和secondupload.image/jpeg。并且第三方读取了您的文件名的最后一段。即jpeg。可能将它们误认为同一个文件。尝试使用pathinfo(storage_path($path), PATHINFO_EXTENSION)代替。

$request = Http::withHeaders(
  [
    'Accept' => 'application/json',
  ]
)
->attach(
  'firstupload',
  Storage::get('/uploads/firstupload.jpeg'),
  'firstupload.' . pathinfo(storage_path('/uploads/firstupload.jpeg'), PATHINFO_EXTENSION),
)
->attach(
  'secondupload',
  Storage::get('/uploads/secondupload.jpeg'),
  'secondupload.' . pathinfo(storage_path('/uploads/secondupload.jpeg'), PATHINFO_EXTENSION),
)
->post(
  'https://thirdpartyapi.com/fileUpload',
  [
    'uploadType' => 'imageUpload',
  ]
);