An exe extension denotes an excutable file like an application. If there is no data in the file to which it can use to open an application then the exe file would attempt to open then close again, that is what is happening.
A txt file is just a notepad sheet with text. You just can't rename a text file with an exe extensions even if it was an exe file to begin with so that the sender could email the file without consequence, the data bits in the header (the first line or two in the file) would be changed by email virus scanners and possibly by the users own operating system.
The sender should have just zipped the exe file and sent it rather than trying to cheat the system.
"fixtheapp" that could mean "fixed the application" which is probably an actual application executable that he has used with a special code to remove something "special" so that it works on your system.....