Symptoms
You have found that a large number of files exist inside of a cPanel user's tmp directory that contain a name similar to the following example:
/home/cpanelusername/tmp/RCMTEMPattmnt60ef155a7d8bd342637901
If you use the file command on the files you may find a variety of file types such as images and mail files:
# file /home/cpanelusername/tmp/RCMTEMPattmnt60ef2fdf349f8506711047
/home/cpanelusername/tmp/RCMTEMPattmnt60ef2fdf349f8506711047: PNG image data, 27 x 28, 8-bit/color RGBA, non-interlaced
# file /home/cpanelusername/tmp/RCMTEMPattmnt60ef15568f9c7805574194
/home/cpanelusername/tmp/RCMTEMPattmnt60ef15568f9c7805574194: SMTP mail, ASCII text, with CRLF line terminators
Description
Roundcube stores email attachments added via the Roundcube interface in the cPanel user's temporary directory. Seeing these files is perfectly normal and is not a cause for concern.
These files are configured to expire after 48 hours and then are removed during nightly maintenance. If you find that these files are building up too quickly and they need to be removed more often, you may use the guide listed below to change the expiry time.
Resolution
How to Configure the Expire Time for Roundcube Temporary Files