fix: respect resource mime type in responses

The server was ignoring mime types set on resources, defaulting to text/plain
for strings and application/octet-stream for bytes. Now properly preserves
the specified mime type in both FastMCP and low-level server implementations.

Note that this is breaks backwards compatibility as it changes the return
values of read_resource() on FastMCP. It is BC compatible on lowlevel since
it only extends the callback.

Github-Issue: #152
Reported-by: eiseleMichael

src/mcp/server/fastmcp/server.py

@@ -197,14 +197,16 @@ class FastMCP:
             for template in templates
         ]
 
-    async def read_resource(self, uri: AnyUrl | str) -> str | bytes:
+    async def read_resource(self, uri: AnyUrl | str) -> tuple[str | bytes, str]:
         """Read a resource by URI."""
+
         resource = await self._resource_manager.get_resource(uri)
         if not resource:
             raise ResourceError(f"Unknown resource: {uri}")
 
         try:
-            return await resource.read()
+            content = await resource.read()
+            return (content, resource.mime_type)
         except Exception as e:
             logger.error(f"Error reading resource {uri}: {e}")
             raise ResourceError(str(e))
@@ -606,7 +608,7 @@ class Context(BaseModel):
             progress_token=progress_token, progress=progress, total=total
         )
 
-    async def read_resource(self, uri: str | AnyUrl) -> str | bytes:
+    async def read_resource(self, uri: str | AnyUrl) -> tuple[str | bytes, str]:
         """Read a resource by URI.
 
         Args: