'_File' is not a subtype of type 'String' in type cast
files in your code looks like plural, but your code doesn't show details.
This is the code to get a single file and this is sent as an array where more than one entry can be added[fileContentBase64] or [file1ContentBase64, file2ContentBase64, file3ContentBase64]
import 'dart:convert';...var fileContent = file.readAsBytesSync();var fileContentBase64 = base64.encode(fileContent); var response = await http.post("http://206.189.92.174:4000/API/Posts/Cube_Post_Submit", body : { 'attachments' : [fileContentBase64] , 'User_Id' : "5acc5d6e1295332c28f7e205", 'Cubes_Id' : jsonstring, 'Post_Text' : "hello", 'Post_Category' : "Story", 'Post_Link': "" } ); print(response.body);