Using the remote/attachable debugger capability of Progress Developer Studio for OpenEdge (PDS for OE), attempting to single step through an application running on a Linux system, if the source file has a non-standard extension, in this case upper case .P, PDS for OE does not show the source lines of the file. Instead an error message is shown, namely, "ABL file not found". Files with an extension of lower case .p, have no trouble having their source code displayed.

1)  On a Linux system startup proenv
2)  cd to the directory that is shared by the linux system and the Windows system
3)  Make the system debuggable , type proDebugEnable -enable-all <this must be done by root>
4)  Start up the application as follows:
      _progres -p <file name of the starting file> -debugReady <debug port to which PDS for OE will attach>
5)  When the pop-up window "Use port <port number from above> for the Debugger to connect to" appears, DO NOT click OK
6)  Shift to the Windows system
7)  In PDS for OE, click on the file that starts the application, this is most likely the file started on the linux system
8)  Click Run-> Debug configurations....
9)  Find Progress External OpenEdge AVM
10) If this is the first time debugging to this remote AVM, click the new icon in the upper left, otherwise use the existing configuration.
11) If not already set, click the "Attach to debug-ready process" radio button.
12) Fill in the host address (of the linux system) and the debug port (set using the value from -debugReady parameter on the Linux system).
13) Click the debug button
14) A pop-up window appears "Application is in alert box. Satisfy the alert box and the debugger will get control at the next executable statement".
15) Click OK on the Windows system's pop-up box.
16) Return to the linux box and click the OK button.
17  Single step through the first file whose extension is .p (lowercase .p). Source code should appear.
18) At a line that runs a file whose extension is an uppercase .P, perform a step into operation. The error "ABL file not found" replaces the displayed source code. 
19) When the single stepping returns to the original file (with extension lowercase .p) source code display returns.

By default Progress Developer Studio recognizes the following extensions only:
.p, .w, .cls, .pgen, .html, .htm

Because of this the file with the P extension was not loaded in the source pane.

Add additional extensions that the debugging session requires for displaying source code in the remote debugger.

The steps for adding additional extensions to PDS for OpenEdge are as follows:

1. Click Window->Preferences->Progress OpenEdge->Editor->Build

2. Add any additional extensions, required by the application, to the end of the list of extensions. 

Again, please be mindful that:

1. Consider the extensions as case sensitive
2. The list is comma separated
3. Do NOT include a period '.' with the letter

References to Other Documentation:

Progress Developer Studio for OpenEdge, Online Help:
https://documentation.progress.com/output/OpenEdge116/pdfs/devstudio/devstudio.pdf

Progress article(s):
 The PDSOE attachable debugger doesn't support custom include file extensions