The X
按字母分类：
extension name file type Classification
U3I U3 Application Information File text file
UNAUTH SiteMinder Unauthorized Message File text file
UNX Unix Text File text file
UOF Uniform Office Document text file
UOT Uniform Office Document text file
UPD Program Update Information text file
UTF8 Unicode UTF8-Encoded Text Document text file
UTXT Unicode Text File text file