AFNetworking中提供的图片上传方法：
[_sessionManager POST:uploadUrl parameters:dict constructingBodyWithBlock:^(id<AFMultipartFormData> _Nonnull formData) {
if (imageData) {
[formData appendPartWithFileData:imageData name:<#name#> fileName:<#fileName#> mimeType:@"jpg"];
}
} success:^(NSURLSessionDataTask * _Nonnull task, id _Nonnull responseObject) {
suc(task,responseObject);
} failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
fail(task,error);
}];
可以看到里边有几个字段：
/**
Appends the HTTP header `Content-Disposition: file; filename=#{filename}; name=#{name}"` and `Content-Type: #{mimeType}`, followed by the encoded file data and the multipart form boundary.
@param data The data to be encoded and appended to the form data.
@param name The name to be associated with the specified data. This parameter must not be `nil`.
@param fileName The filename to be associated with the specified data. This parameter must not be `nil`.
@param mimeType The MIME type of the specified data. (For example, the MIME type for a JPEG image is image/jpeg.) For a list of valid MIME types, see http://www.iana.org/assignments/media-types/. This parameter must not be `nil`.
*/
- (void)appendPartWithFileData:(NSData *)data
name:(NSString *)name
fileName:(NSString *)fileName
mimeType:(NSString *)mimeType
{
NSParameterAssert(name);
NSParameterAssert(fileName);
NSParameterAssert(mimeType);
NSMutableDictionary *mutableHeaders = [NSMutableDictionary dictionary];
[mutableHeaders setValue:[NSString stringWithFormat:@"form-data; name=\"%@\"; filename=\"%@\"", name, fileName] forKey:@"Content-Disposition"];
[mutableHeaders setValue:mimeType forKey:@"Content-Type"];
[self appendPartWithHeaders:mutableHeaders body:data];
}
而 Appends the HTTP header Content-Disposition: file; filename=#{filename}; name=#{name}" and Content-Type: #{mimeType}, followed by the encoded file data and the multipart form boundary.又是什么呢？
可以看出来：
name：就是前后端约定好的描述图片的字段，（比如传入avatar，那就是代表该图片是用户头像，后台在取的时候，也需要根据这个name来取出）
fileName: 上传的文件名称
https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Headers/Content-Disposition
https://tools.ietf.org/html/rfc6266
