Adversaries may communicate using application layer protocols associated with transferring files to avoid detection/network filtering by blending in with existing traffic. Commands to the remote system, and often the results of those commands, will be embedded within the protocol traffic between the client and server. 

Protocols such as SMB(Citation: US-CERT TA18-074A), FTP(Citation: ESET Machete July 2019), FTPS, and TFTP that transfer files may be very common in environments.  Packets produced from these protocols may have many fields and headers in which data can be concealed. Data could also be concealed within the transferred files. An adversary may abuse these protocols to communicate with systems under their control within a victim network while also mimicking normal, expected traffic. 
File Transfer Protocols
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1071/002  external_id: T1071.002     source_name: ESET Machete July 2019  description: ESET. (2019, July). MACHETE JUST GOT SHARPER Venezuelan government institutions under attack. Retrieved September 13, 2019.  url: https://www.welivesecurity.com/wp-content/uploads/2019/08/ESET_Machete.pdf     source_name: University of Birmingham C2  description: Gardiner, J.,  Cova, M., Nagaraja, S. (2014, February). Command & Control Understanding, Denying and Detecting. Retrieved April 20, 2016.  url: https://arxiv.org/ftp/arxiv/papers/1408/1408.1136.pdf     source_name: US-CERT TA18-074A  description: US-CERT. (2018, March 16). Alert (TA18-074A): Russian Government Cyber Activity Targeting Energy and Other Critical Infrastructure Sectors. Retrieved June 6, 2018.  url: https://www.us-cert.gov/ncas/alerts/TA18-074A     