I have seen fair amount of threads regarding bad request errors upon file upload, however unlike for others, I am sure that Spring is not at fault here, as I am able to upload file via curl without an issue: curl -X POST -v http://localhost:8080/rest/datasets/  -T test.xlsx
ExtJS uploader:
{
            xtype: 'filefield',
            fieldLabel: 'Select file',
            name: 'file',
            fieldName : 'file',
            listeners: {
                change: function(filefield, value, eOpts) {
            var form = filefield.up('form').getForm();
            form.submit({
                url: '/rest/datasets',
                headers : {
                    'Accept' : '*/*',
                    'Content-Type' : 'multipart/form-data'
                },
                waitMsg: 'Uploading'
                });
                }
            }
        }
Spring controller
@RestController
@RequestMapping("rest/datasets")
public class DatasetController {
    @RequestMapping(method = RequestMethod.POST)
    public String uploadFile(
        @RequestParam("file") MultipartFile file) {
        ...
    }
}
I am using ExtJS 6.0.1 and Spring Boot 1.3.3