JabRef version 5.0 on Ubuntu 16.04, and on Windows 10, both installed using the installation files (.deb or .msi) downloaded from the JabRef website.
I assume the Java version will be that provided by the installer.

• I have NOT tested the latest development version from http://builds.jabref.org/master/ as I do not have the capabilities needed to do this. I have searched for other related Bug reports.
  The most relevant seem to be:
  Field properties - future implementation #978 Field properties - future implementation
  Sorting by title is broken #2200 Sorting by title is broken
  but both of these issues have been "Closed"

JabRef allows the user to add custom general entry fields that contain numerical values. All versions of JabRef prior to 5.0 allowed the Entry Table to be sorted in the numerical order of the value of such a custom field, simply by clicking on the column header.

However, with JabRef 5.0, the numerical (integer) values in this column are sorted in alphabetical order instead!
This means, for example that the numbers 1,2,3,10,13, 21
are sorted into the order 1,10,13,2,21,3
This suggests the numerical values are being treated as plain text.
I did not observe this problem with JabRef 4.3.1 or earlier versions.

Steps to reproduce the behavior:

1. Add a custom entry field to a BibTeX database using a JabRef version earlier than 5.0.
2. Populate this field with a range of positive integer values.
3. Open the BibTeX database file with JabRef 5.0.
4. Sort the Entry Table by the values in the Custom Entry field, by clicking on the relevant column header. The Table rows will be sorted by the alphabetical order of the number, not the numerical value.

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