Guess image mime type from file extension (fixes #5196)

crates/api_common/Cargo.toml

@@ -64,6 +64,7 @@ actix-web = { workspace = true, optional = true }
 enum-map = { workspace = true }
 urlencoding = { workspace = true }
 mime = { version = "0.3.17", optional = true }
+mime_guess = "2.0.5"
 webpage = { version = "2.0", default-features = false, features = [
   "serde",
 ], optional = true }

crates/api_common/src/request.rs

@@ -64,12 +64,21 @@ pub async fn fetch_link_metadata(url: &Url, context: &LemmyContext) -> LemmyResu
     .await?
     .error_for_status()?;
 
-  let content_type: Option<Mime> = response
+  let mut content_type: Option<Mime> = response
     .headers()
     .get(CONTENT_TYPE)
     .and_then(|h| h.to_str().ok())
     .and_then(|h| h.parse().ok());
 
+  // In some cases servers send a wrong mime type for images, which prevents thumbnail
+  // generation. To avoid this we also try to guess the mime type from file extension.
+  let guess = mime_guess::from_path(url.path());
+  if let Some(guess) = guess.first() {
+    if guess.type_() == mime::IMAGE {
+      content_type = Some(guess);
+    }
+  }
+
   let opengraph_data = {
     // if the content type is not text/html, we don't need to parse it
     let is_html = content_type