Bug 29755 Attach source does not allow one to select a folder... [build path]
When opening some .class that does not yet have a source attachment eclipse shows a text version of the interface defintion and a button for Attaching source. When pressing this button a window appaer where one can enter the name/location of a folder, zip or jar for the source. There are also a button for choosing it via a file selection dialog and this works fine for zip and jar files - but not for selecting a directory :( Workaround is just to enter the directory manually.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/SourceAttachmentBlock.java
if (fFileVariablePath == null) {
					status.setError(NewWizardMessages.getString("SourceAttachmentBlock.filename.error.varnotexists")); 
					return status;
				}
				resolvedPath= fFileVariablePath.append(filePath.removeFirstSegments(1));
				
				if (resolvedPath.isEmpty()) {
					status.setWarning(NewWizardMessages.getString("SourceAttachmentBlock.filename.warning.varempty")); 
					return status;
				}
				File file= resolvedPath.toFile();
				if (!file.exists()) {				
					String message= NewWizardMessages.getFormattedString("SourceAttachmentBlock.filename.error.filenotexists", resolvedPath.toOSString()); 
					status.setWarning(message);
					return status;
				}						
				fResolvedFile= file;
				
			} else {
				File file= filePath.toFile();
				IResource res= fWorkspaceRoot.findMember(filePath);
				if (res != null && res.getLocation() != null) {
					file= res.getLocation().toFile();
				}
				if (!file.exists()) {
					String message=  NewWizardMessages.getFormattedString("SourceAttachmentBlock.filename.error.filenotexists", filePath.toString()); 
					status.setError(message);
					return status;
				}
				fResolvedFile= file;