refactor(ConnectionManager): simplify hasValidTransferFileExtension using getTransferFileType

[bobtista]

After #1819 is merged, hasValidTransferFileExtension in ConnectionManager.cpp can be simplified.

Currently it maintains its own hardcoded validExtensions[] array and loops over it manually. PR #1819 adds getTransferFileType() which already does the same extension lookup against the transferFileRules[] table.

hasValidTransferFileExtension should be refactored to call getTransferFileType() and return getTransferFileType(ext) != TransferFileType_Invalid, eliminating the duplicate extension list and the redundant loop.

Noted by xezon in the #1819 review.