editing: Decide editable based on true mime type

Previously it was decided on the early unreliable mime type.

Fixes #449

src/metadata.rs

@@ -105,17 +105,28 @@ impl Metadata {
     }
 
     pub fn mime_type(&self) -> Option<String> {
-        self.file_info
-            .as_ref()
-            .and_then(|x| x.mime_type.as_ref())
-            .map(|x| x.to_string())
+        self.mime_type.clone()
+    }
+
+    /// Unreliable mime type
+    ///
+    /// Before the image is successfully loaded, this mime type might be wrong
+    /// since it is not yet based on the more complex mime type detection used
+    /// by glyin.
+    pub fn unreliable_mime_type(&self) -> Option<String> {
+        self.mime_type().or_else(|| {
+            self.file_info
+                .as_ref()
+                .and_then(|x| x.mime_type.as_ref())
+                .map(|x| x.to_string())
+        })
     }
 
     pub fn format_name(&self) -> Option<String> {
         self.image_info
             .as_ref()
             .and_then(|x| x.format_name.clone())
-            .or_else(|| self.mime_type())
+            .or_else(|| self.unreliable_mime_type())
     }
 
     pub fn alpha_channel(&self) -> Option<bool> {