def define_file_extensions(self):
all_a_extensions = []<EOL>all_b_extensions = []<EOL>all_c_extensions = []<EOL>all_d_extensions = []<EOL>all_extensions = {MULTIFILE_EXT}<EOL>for a in [False, True]:<EOL><INDENT>for b in [False, True]:<EOL><INDENT>for c in [False, True]:<EOL><INDENT>for d in [False, True]:<EOL><INDENT>ext = '<STR_LIT:.>'<EOL>if a:<EOL><INDENT>ext += '<STR_LIT:a>'<EOL><DEDENT>if b:<EOL><INDENT>ext += '<STR_LIT:b>'<EOL><DEDENT>if c:<EOL><INDENT>ext += '<STR_LIT:c>'<EOL><DEDENT>if d:<EOL><INDENT>ext += '<STR_LIT:d>'<EOL><DEDENT>print(ext)<EOL>if a or b or c or d:<EOL><INDENT>all_extensions.add(ext)<EOL><DEDENT>if a:<EOL><INDENT>all_a_extensions.append(ext)<EOL><DEDENT>if b:<EOL><INDENT>all_b_extensions.append(ext)<EOL><DEDENT>if c:<EOL><INDENT>all_c_extensions.append(ext)<EOL><DEDENT>if d:<EOL><INDENT>all_d_extensions.append(ext)<EOL><DEDENT><DEDENT><DEDENT><DEDENT><DEDENT>return all_a_extensions, all_b_extensions, all_c_extensions, all_d_extensions, all_extensions<EOL>
Defines a couple file extensions, that will be used in the whole test class.
They are defined by doing all the 1-, 2-, 3-, and 4- combinations of a,b,c,d.
This method returns lists containing all extensions containing 'a', all extensions containing 'b', etc.

'.a', '.b', '.c', '.d', 'ab', 'ac', 'ad' ..., '.abc', '.bcd', ... '.abcd'
:return: