fix: remove MIME type validation from MCPServer Resource

The regex pattern on Resource.mime_type was the only MIME type validation
anywhere in the MCP ecosystem. It rejected valid RFC 2045 MIME types such
as quoted parameter values (text/plain; charset="utf-8") and was
inconsistent with ResourceTemplate in the same module, the protocol-level
types in _types.py, and every other SDK.

The MCP spec defines mimeType as an unconstrained optional string.

Github-Issue: #1756

Author: maxisbey

src/mcp/server/mcpserver/resources/base.py

@@ -23,11 +23,7 @@ class Resource(BaseModel, abc.ABC):
     name: str | None = Field(description="Name of the resource", default=None)
     title: str | None = Field(description="Human-readable title of the resource", default=None)
     description: str | None = Field(description="Description of the resource", default=None)
-    mime_type: str = Field(
-        default="text/plain",
-        description="MIME type of the resource content",
-        pattern=r"^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+(;\s*[a-zA-Z0-9\-_.]+=[a-zA-Z0-9\-_.]+)*$",
-    )
+    mime_type: str = Field(default="text/plain", description="MIME type of the resource content")
     icons: list[Icon] | None = Field(default=None, description="Optional list of icons for this resource")
     annotations: Annotations | None = Field(default=None, description="Optional annotations for the resource")
     meta: dict[str, Any] | None = Field(default=None, description="Optional metadata for this resource")

tests/server/mcpserver/resources/test_resources.py

@@ -91,6 +91,14 @@ def dummy_func() -> str:  # pragma: no cover
         )
         assert resource.mime_type == "application/json"
 
+        # RFC 2045 quoted parameter value (gh-1756)
+        resource = FunctionResource(
+            uri="resource://test",
+            fn=dummy_func,
+            mime_type='text/plain; charset="utf-8"',
+        )
+        assert resource.mime_type == 'text/plain; charset="utf-8"'
+
     @pytest.mark.anyio
     async def test_resource_read_abstract(self):
         """Test that Resource.read() is abstract."""