Missing record when file deleted

[abcwers]

I am check HTML source for some errors, like links to files with extension .pdf .docx .xlsx have original link, but not to local files. it's bug #22

and found new bug witch some link to files for example [ files/C02NU3NPK7T/F02P30C9QTE .undefined ] i am check it in local storage, it's exist file with undefined extension, or not. In real file is not exist. After i am check original SLACK chat record, here i found, that in slack is record, This file was deleted. This text record is not exist in slack backup HTML code, only not correct link to not existing file. It's maybe useful record, if you have no backup it before. You will know, that was some file before deleted and missing.

So need add this feature for correct view saved backup.