isRelativeURL currently misclassifies several URL/path inputs in Rocket.Chat.

Current implementation:

export const isRelativeURL = (str: string): boolean => /^[^/]+/[^/].$|^/[^/].$/gim.test(str);

This causes incorrect results:

• "test" returns false (should be treated as relative path)
• "." returns false (should be treated as relative path)
• "data:image/gif;base64,..." returns true (should not be treated as relative URL)

There are already TODOs in unit tests indicating this mismatch:

/C:/Users/evilt/Documents/GitHub/Rocket.Chat/apps/meteor/tests/unit/lib/utils/isRelativeURL.spec.ts:8

• /C:/Users/evilt/Documents/GitHub/Rocket.Chat/apps/meteor/tests/unit/lib/utils/isRelativeURL.spec.ts:10
• /C:/Users/evilt/Documents/GitHub/Rocket.Chat/apps/meteor/tests/unit/lib/utils/isRelativeURL.spec.ts:13

This validator is used in message URL validation (validPartialURLParam), so wrong classification can affect which
values are accepted/rejected in message payload fields:

• /C:/Users/evilt/Documents/GitHub/Rocket.Chat/apps/meteor/app/lib/server/functions/sendMessage.ts:47

Expected behavior:

• Relative paths like test, ., ./test, ../test, /test should be handled consistently as relative.
• Scheme-based URLs like https://..., data:..., javascript:... should not be considered relative.

Start with apps/meteor/tests/unit/lib/utils/isRelativeURL.spec.ts and inspect the isRelativeURL implementation, then read its use in apps/meteor/app/lib/server/functions/sendMessage.ts. Run the targeted unit test and verify that relative paths and dot paths are accepted while scheme-based URLs such as https:, data:, and javascript: are rejected.