How do you upload a file with text data in Retrofit?

What's really being asked

Whether you understand how Retrofit 2 sends mixed binary and text payloads using multipart form-data. Interviewers want to see that you distinguish between JSON request bodies and file uploads, and that you can name the exact annotations and types required without guessing.

The full answer

First, annotate the interface method with @POST and @Multipart. The @Multipart annotation tells Retrofit to build a multipart form-data request body. Second, the file parameter must be declared as @Part MultipartBody.Part. You create this by wrapping a File in a RequestBody with the correct media type such as image/jpeg, then passing it to MultipartBody.Part.createFormData along with the form field name and optional filename. Third, text fields are passed as @Part RequestBody or directly as @Part String if no special media type is needed. Fourth, note that you should not manually set the Content-Type header to multipart/form-data because Retrofit adds it automatically along with the boundary when @Multipart is present. Fifth, mention that each @Part takes a name string that must match what the backend expects.

The mistakes people make

Using @Field or @FormUrlEncoded instead of @Part and @Multipart. Form-urlencoded data cannot carry binary file streams. Proposing a single @Body with a Kotlin data class; that sends application/json and will fail when the server expects multipart. Passing a raw File object without wrapping it in RequestBody and MultipartBody.Part. Forgetting the @Multipart annotation, which causes the request to omit the required boundary and content type headers.

What usually comes next

How would you upload multiple files at once? Expect the candidate to suggest multiple @Part parameters or a @PartMap. How do you track upload progress? Expect an answer about a custom RequestBody that overrides writeTo and reports bytes written, or an OkHttp network interceptor. What if the server requires a specific filename in the multipart payload? Expect MultipartBody.Part.createFormData with an explicit filename argument rather than relying on the file name property.

A concrete example

The interface declares @Multipart @POST profile upload fun uploadProfile @Part image MultipartBody.Part @Part userId RequestBody Call ResponseBody. To call it, create a File from the local path, wrap it with file.asRequestBody passing image/jpeg, then build MultipartBody.Part.createFormData with the server field name profile image, the filename, and the request body. For the user ID, convert the string with userId.toRequestBody using text/plain. Pass both parts to the method and enqueue or await the call.