fix: reset MIME type if passed to set_file value is None

src/message.rs

@@ -980,19 +980,14 @@ impl Message {
             }
         }
         self.param.set(Param::File, file);
-        if let Some(filemime) = filemime {
-            self.param.set(Param::MimeType, filemime);
-        }
+        self.param.set_optional(Param::MimeType, filemime);
     }
 
     /// Set different sender name for a message.
     /// This overrides the name set by the `set_config()`-option `displayname`.
     pub fn set_override_sender_name(&mut self, name: Option<String>) {
-        if let Some(name) = name {
-            self.param.set(Param::OverrideSenderDisplayname, name);
-        } else {
-            self.param.remove(Param::OverrideSenderDisplayname);
-        }
+        self.param
+            .set_optional(Param::OverrideSenderDisplayname, name);
     }
 
     /// Sets the dimensions of associated image or video file.

src/param.rs

@@ -281,6 +281,16 @@ impl Params {
         self
     }
 
+    /// Sets the given key from an optional value.
+    /// Removes the key if the value is `None`.
+    pub fn set_optional(&mut self, key: Param, value: Option<impl ToString>) -> &mut Self {
+        if let Some(value) = value {
+            self.set(key, value)
+        } else {
+            self.remove(key)
+        }
+    }
+
     /// Check if there are any values in this.
     pub fn is_empty(&self) -> bool {
         self.inner.is_empty()