Hi
If you download your application.xyz via a browser or ftp to you documents folder, by-passing your 'utilty'
then check its size and upload it to VirusTotal , to see if any virus engine flags anything.
if its locatable via https, you can give VirusTotal the link and it may hi-light something else.
If all that is fine, then look at your utility, it could be something simple ie your reading the stream as a block of 64MB and writing to disk, but now your xyz has exceeded 64Mb is size and its not completing the following blocks, ie truncates the size.
Do not upload your file to VirusTotal if it contains sensitive, private, or proprietary data. VirusTotal shares uploaded files with the global cybersecurity community for analysis. But if its for other users to download, this shouldn’t be a problem.
addition, make sure your web server hasn’t had any updates recently, ie, changing php version, each php version has a maximum file download size, set in its ini file.