Resource Mime Type is ignored

[eiseleMichael]

Describe the bug
Setting a mime type on a Ressource using FastMCP is being ignored by the low level server as the returned type is based on the ReadResourceRequest result type.

To Reproduce
Create any resource that returns a base64 string and explicitly sets a mime type, e.g.:

@mcp.resource("hackernews://get_logo", mime_type="image/png")
def get_logo() -> str:
    """Returns the Hacker News logo."""
    with open("../data/logo.png", "rb") as image_file:
        image_bytes = image_file.read()
        base64_string = base64.b64encode(image_bytes).decode('utf-8')
        return base64_string

Invoke either directly or using the MCP inspector

{
  "method": "resources/read",
  "params": {
    "uri": "hackernews://get_logo"
  }
}

Actual behavior
It returns

{
  "contents": [
    {
      "uri": "hackernews://get_logo",
      "mimeType": "text/plain,
      "blob": "aVZCT1J3MEtHZ29B [...]
}

Note: a resource that it as bytes does not make much of a difference, it just changes the mime type to the very generic octet-stream.

{
  "contents": [
    {
      "uri": "hackernews://get_logo",
      "mimeType": "application/octet-stream",
      "blob": "aVZCT1J3MEtHZ29

Expected behavior
It should respect the mime type for both string and bytes content.

If you need a repo to reproduce it, I'm happy to help.
MCP version 1.2.0.