Commented Unassigned: Broken XLSX [13792]

Hi, I have several problematic XLSX files. When I try to open and save them result file is broken and can not be opened by excel.
Can you tell me what is wrong with the files ?

Thanks
Comments: ** Comment from web user: Kelemen **

Hi, I have found where is the problem (at least one):
1) Open empty workbook and type char & into cell A1.
2) Type formula A1 & A1 into cell B1.
3) Open and save using NPOI. Result is unreadable.