Case 1
Let's see the  code shown in the following: 
- string rootPath = ConfigurationManager.AppSettings["FilePath"];  
- var currentApplicationPath = HttpContext.Current.Request.PhysicalApplicationPath;  
- var provider = new MultipartFormDataStreamProvider(currentApplicationPath + "\\tempUploads");  
- try{  
-   
-      await Request.Content.ReadAsMultipartAsync(provider);  
-   
-    }  
- catch(Exception ex)  
-     
- {  
-   
-    throw ex;  
-   
- }  
Here i am passing from header Accept: tex/json.
I am getting error. with used above code, error shown in the following  screenshot:
 Case 2
If I used MultipartFormDataStreamProvider  with app  ConfigurationManager.AppSettings["FilePath"]  than  I was successfully  execute my code.
- string rootPath = ConfigurationManager.AppSettings["FilePath"];  
- var provider = new MultipartFormDataStreamProvider(ConfigurationManager.AppSettings["FilePath"])  
-   
- try{  
-   
-     await Request.Content.ReadAsMultipartAsync(provider);  
-   
- }  
- catch(Exception ex)  
-   
- {  
-   
-    throw ex;  
-   
- }