diff --git a/internal/apiform/encoder.go b/internal/apiform/encoder.go
index 58905eab..555330ae 100644
--- a/internal/apiform/encoder.go
+++ b/internal/apiform/encoder.go
@@ -319,7 +319,7 @@ func (e *encoder) newReaderTypeEncoder() encoderFunc {
 			filename = path.Base(named.Name())
 		}
 		if typed, ok := reader.(interface{ ContentType() string }); ok {
-			contentType = path.Base(typed.ContentType())
+			contentType = typed.ContentType()
 		}
 
 		// Below is taken almost 1-for-1 from [multipart.CreateFormFile]