[feature]: Valid file uploads (CSV, MD) failing with "Invalid file type" due to strict browser MIME matching

Is there an existing issue for this? • I have searched the existing issues Summary Feature Request: Expand Allowed Attachment File Types & Improve MIME Type Validation Description Currently, the self-hosted version of Plane implements a very strict whitelist for file uploads via the ATTACHMENT_MIME_TYPES setting in plane/settings/common.py. While this provides security, it results in a poor user experience because: 1. Many standard file types are missing from the list. 2. Different operating systems and browsers often send slightly different MIME type strings for the exact same file extension (e.g., .md sent as text/plain or text/x-markdown, .csv sent as application/csv). When a user tries to upload a perfectly safe, standard file, if their browser's inferred MIME type doesn't perfectly match Plane's hardcoded list, they receive a generic {"error": "Invalid file type.", "status": False} response. Proposed Solution We propo…