Sorry for being a bit of a dick, I think you mean that the file “is 3.8MB”.
“mb” would mean millibit, 3.8millibit is an impossibly small file size, and would never exist practically (though I an sure that with some clever maths a zip bomb could be designed so that one bit of data could be compressed into 3.8millibits)
MB is the proper shorthand for MegaByte, a decent file size for a high quallity pucture, depending on the format and compression.
Unless we analyze the image, and determine the image format and compression settings we have no idea of if 3.8MB is a resonable size of the file or not, and the mods have hidden a rar file in the picture file, it is highly improbable that would be the case however.