fix the comment in File class in utilities

[Anurag-gg]

Description

File class is a helper class for returning file data (and not audio) from tools
https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/utilities/types.py#L295

Example Code

Version Information

FastMCP version:                                               2.12.3
MCP version:                                                   1.14.0
Python version:                                                3.13.7
Platform:          Linux-6.16.8-200.fc42.x86_64-x86_64-with-glibc2.41

[Anurag-gg]

#2066

[jlowin]

Closed in #2066