Unknown file type containing a virus I will try to keep the story as short as possible just so you can have a more clear view of what is my concern. While, I was browsing the Internet on my phone using the Chrome Browser App, I got a notification that I am downloading a file. The notification showed that the file which was being downloaded is an MP3. 

Short after that before I even had the time to react and stop the downloading process the file was already on my phone. I immediately deleted it from the Download folder using the File Manager on my phone. However, I noticed that the file format was missing. The file name didn't have any ending such as .pdf, .mp3, .apk, etc. 

After that I scanned my phone with several anti-virus software apps and all returned that my phone is safe. However, I am not really sure if I can fully trust them. 

I decided to put my phone on quarantine until I got more information regarding this problem. I turned off my Wi-Fi and removed the SIM card from it just in case if it is an actual virus to prevent it sending data or messages to an unknown direction. 

I have the following questions:


If the file format was unknown to the system does this mean that the system was not able to execute the file?
If I never tried to open the file does this mean that the file was never been executed?
If I delete the file can I say that I fixed the problem or I should actually do question 4?
Should I return the factory settings on my phone, so I can wipe everything and have a clean device once again?


PS: I will be very grateful to everyone, who share their thoughts about this with me. Thank you in advance.
File extensions tend to be present only as mnemonics for the user to quickly identify a file, and for software to filter out incompatible files prior to loading and opening them. The latter explains why your OS did not recognize the file type.

To execute a file is common lingo, even if inexact: you execute a program, which then tries to parse the file. Should this program fail to parse the file, it will either report the error or crash. This means that you're probably safe.

Second, Android employs a deal of security mechanisms in order to limit the execution of possibly malicious software. For example, malware tend to come in the form of binary files. On modern Android versions, such files (as well as plaintext scripts) cannot be launched from neither the internal or the external storage (if any). This means, that the only kind of file that could harm your device are APK files that have been installed – namely, apps.

Even then, apps are restricted in many a way, as each of them run in an isolated context, being able to communicate with others only via predefined means. Therefore, the only way some code could do harm to your device, is if the operating system itself presents bugs that can be exploited, and such issues stem from OSes that haven't been updated in a while. Indeed, even other apps could be exploitable, but for an attacker to make use of such a weakness, they would need prior knowledge of your installed software (and even then, they could only gain access to what the vulnerable app itself can access).

If I were you, rather than worrying about having been infected, I would be concerned about my browser downloading files without my expressed authorization. You needn't do a factory reset, but you may want to replace your browser if it continues in its behavior.