Sort by Extension
You are given a sequence of files as strings. You need to sort this the sequence by the file extension. The files with the same extension (or without one) should be sorted by name.
Some possible cases:
- Filename cannot be an empty string;
- Sorting order: files without name, files...
- Filename like str1.str2.str3 has an extension str3 and a name str1.str2;
- Filename like .str1.str2 has an extension str2 and a name .str1.
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